Pump unit

ABSTRACT

The present invention provides a pump unit including at least one hydraulic pump with inlet and outlet ports, a pump case for accommodating the hydraulic pump having an opening through which the hydraulic pump is insertable, and a lid, called a center section, closing the pump case. The center section forms a pair of inlet/outlet passages communicating an end with the inlet and outlet ports of the hydraulic pump and an end opening through a pump case abutting surface of the center section, and a first charging passage by which hydraulic fluid is fed through the pump case abutting surface of the center section. At least one of the pump case and the center section communicates the first charging passage with the pair of inlet/outlet passages via a first hydraulic fluid feeding valve preventing the reverse flow into said first charging passage.

BACKGROUND OF THE INVENTION

The present invention relates to a pump unit used for various purposes.

More particularly, a first aspect of the present invention relates to apump unit that includes a hydraulic pump, a pump case for accommodatingthe hydraulic pump and a center section connected to the pump case.

A hydraulic pump is used in various applications and in particular asthe hydraulic pump adapted for operation in association with an actuatordriven through the hydraulic effect. In this case, the hydraulic pump isconnected to the actuator via a pair of hydraulic lines, and the outputflow rate of hydraulic fluid discharged from the hydraulic pump isvaried to cause the pressure difference between the pair of hydrauliclines, thereby driving the actuator. When the hydraulic pump is thusconnected to the actuator via the pair of hydraulic lines so as toconstitute a closed circuit, a charging mechanism is generally requiredto feed pressurized hydraulic fluid to the pair of hydraulic lines.

Specifically, the charging mechanism as required necessarily includes acharge line having a first end through which pressurized hydraulic fluidis fed into the pair of hydraulic lines, and a second end communicatingwith the pair of hydraulic lines, a checking valve for allowing thepressurized hydraulic fluid to flow from the charge line to the pair ofhydraulic lines, while preventing the reverse flow.

For the hydraulic pump with the charging mechanism, it is desirable toreduce machining works for the reduction of the manufacturing cost,and/or improve an assembling efficiency in installing the check valve,or other works. However, there have not been made effective proposals inview of those points.

The first aspect of the present invention has been therefor conceived inconsideration of the prior arts. It is an object of the first aspect ofthe present invention to provide a pump unit with the charging mechanismfor feeding additional hydraulic fluid, which pump is used inassociation with the actuator driven through the hydraulic effect, andis capable of lowering the manufacturing cost and improving theassembling efficiency.

A second aspect of the present invention relates to a pump unit withfirst and second hydraulic pumps that is designed to be operated inassociation with an actuator driven through a hydraulic effect.

A hydraulic pump is used in various applications and in particular asthe hydraulic pump adapted for operation in association with an actuatordriven through the hydraulic effect. The description will hereinafter bemade for the pump unit by taking for example the case where it includesthe first and second hydraulic motors serving as the actuators thatrespectively drive the right and left drive wheels.

For example, U.S. Pat. No. 4,920,733 discloses a vehicle including firstand second hydraulic pumps respectively connected via first and secondhydraulic lines to the first and second hydraulic motors for driving theright and left drive wheels. In this vehicle, the first and secondhydraulic motors respectively have outputs variable in response to theadjustment of the input/output flow rates of the first and secondhydraulic pumps, thereby controlling the rotational speed and rotationaldirection of the right and left drive wheels.

The vehicle of the above arrangement has the first hydraulic pump andthe second hydraulic pump separately arranged from one another, theformer being operated in association with the first hydraulic motor, andthe latter being operated in association with the second hydraulicmotor. Such a separate arrangement of the hydraulic pumps involves atroublesome piping work between the first and second hydraulic pumps andthe first and second hydraulic motors, a troublesome assembling work ofthe pump unit, and pose various other problems.

The second aspect of the present invention has been therefor conceivedin consideration of the above prior art. It is an object of the secondaspect of the present invention to provide a pump unit with the firstand second hydraulic pumps that is capable of achieving thesimplification of the piping work between the actuator and the hydraulicpumps, and the assembling work of the pump unit.

A third aspect of the present invention relates to a pump unit used fora vehicle with first and second hydraulic motors respectively connectedto the right and left drive wheels, and includes first and secondhydraulic pumps that are designed to be respectively operated inassociation with first and second hydraulic motors.

There are known arrangements of the above type described in, forexample, U.S. Pat. No. 4,920,733. According to this U.S. patent, avehicle with first and second hydraulic motors respectively connected tothe right and left drive wheels includes first and second hydraulicpumps respectively operable in association with the first and secondhydraulic motors. In this vehicle, the first and second hydraulic motorsrespectively have outputs variable in response to the adjustment, of theinput/output flow rates of the first and second hydraulic pumps, therebycontrolling the rotational speed and rotational direction of the rightand left drive wheels.

The vehicle of the above arrangement has the first hydraulic pump andthe second hydraulic pump which are separately arranged from oneanother, the former being operated in association with the firsthydraulic motor, and the latter being operated in association with thesecond hydraulic motor. Such a separate arrangement of the hydraulicpumps poses various problems, such as troublesome mounting operation ofthe hydraulic pumps on the vehicle, and troublesome assemblingoperation.

The third aspect of the present invention has been therefor conceived inconsideration of the above prior art. It is an object of the thirdaspect of the present invention to provide a pump unit used for thevehicle with the first and second hydraulic motors respectivelyconnected to the right and left drive wheels, and including first andsecond axial piston pumps of a variable displacement type that iscapable of having improved efficiencies in mounting the pump unit on thevehicle, and improved assembling efficiency.

A fourth aspect of the present invention relates to a pump unit withfirst and second hydraulic pumps that are respectively connected viafirst and second hydraulic lines to first and second actuators driventhrough a hydraulic effect.

A hydraulic pump is used in various applications and in particular asthe hydraulic pump adapted for operation in association with an actuatordriven through the hydraulic effect. The description will hereinafter bemade for the pump unit by taking for example the case where it includesthe first and second hydraulic motors serving as the actuators thatrespectively drive the right and left drive wheels.

For example, U.S. Pat. No. 4,920,733 discloses a vehicle including firstand second hydraulic pumps respectively connected via first and secondhydraulic lines to the first and second hydraulic motors for driving theright and left drive wheels. In this vehicle, the first and secondhydraulic motors respectively have outputs variable in response to theadjustment of the input/output flow rates of the first and secondhydraulic pumps, thereby controlling the rotational speed and rotationaldirection of the right and left drive wheels.

The vehicle disclosed in the above cited U.S. patent has the firsthydraulic pump and the second hydraulic pump separately arranged fromone another, the former being operated in association with the firsthydraulic motor, and the latter being operated in association with thesecond hydraulic motor. Such a separate arrangement of the hydraulicpumps invites a complicated structure of a Weeding passage for feedingworking hydraulic fluid from a reservoir tank to the first hydraulicline and the second hydraulic line, and poses various other problems.

The fourth aspect of the present invention has been therefor conceivedin consideration of the above prior art. It is an object of the fourthaspect of the present invention to provide a pump unit with the firstand second hydraulic pumps that are respectively connected via the firstand second hydraulic lines to the first and second actuators driventhrough the hydraulic effect, and that is capable of achieving asimplified structure of the feeding passage for feeding workinghydraulic fluid to the hydraulic lines extending between the actuatorsand the hydraulic pumps.

A fifth aspect of the present invention relates to a pump unit withfirst and second hydraulic pumps that are respectively connected viafirst and second hydraulic lines to first and second actuators driventhrough a hydraulic effect.

A hydraulic pump is used in various applications and in particular asthe hydraulic pump adapted for operation in association with an actuatordriven through the hydraulic effect. The description will hereinafter bemade for the pump unit by taking for example the case where it includesthe first and second hydraulic motors serving as the actuators thatrespectively drive the right and left drive wheels.

For example, U.S. Pat. No. 4,920,733 discloses a vehicle including firstand second hydraulic pumps respectively connected via first and secondhydraulic lines to the first and second hydraulic motors for driving theright and left drive wheels. In this vehicle, the first and secondhydraulic motors respectively have outputs variable in response to theadjustment of the input/output flow rates of the first and secondhydraulic pumps, thereby controlling the rotational speed and rotationaldirection of the right and left drive wheels.

The vehicle disclosed in the above cited U.S. patent has the firsthydraulic pump and the second hydraulic pump separately arranged fromone another, the former being operated in association with the firsthydraulic motor, and the latter being operated in association with thesecond hydraulic motor. Such a separate arrangement of the hydraulicpumps invites a complicated structure of a feeding passage for feedingworking hydraulic fluid for an HST (hydrostatic transmission) from ahydraulic fluid tank to the first hydraulic line and the secondhydraulic line, and poses various other problems.

As a further disadvantage, the working hydraulic fluid between thehydraulic pumps and the actuators may increase in temperature due to theload from the outside. Such an increase in temperature of the workinghydraulic fluid may invite various problems such as lowering of thevolumetric efficiency, or lowering of the axle revolution speed if thehydraulic motors are used as the actuators for driving the drive wheelsof the vehicle, deteriorating of the curability. However, theabove-cited U.S. patent does not teach any solutions to limit thetemperature of the working hydraulic fluid of the HST.

The fifth aspect of the present invention has been therefor conceived inconsideration of the above prior art. It is an object of the fifthaspect of the present invention to provide a pump unit with the firstand second hydraulic pumps that are respectively connected via the firstand second hydraulic lines to the first and second actuators driventhrough the hydraulic effect, and that is capable of effectivelylimiting the increase in temperature of the working hydraulic fluid tobe replenished to the hydraulic lines between the actuators and thehydraulic pumps.

SUMMARY OF THE INVENTION

According to the first aspect of the present invention, there isprovided a pump unit that includes at least one hydraulic pump withinlet an outlet ports formed therein, a pump case for accommodating theat least one hydraulic pump, the pump case having an opening throughwhich the at least one hydraulic pump is insertable into the pump case,and a lid (also referred to herein as a center section) connected to thepump case in such a manner as to close the opening of the pump case. Thecenter section forms a pair of inlet/outlet passages having first endsrespectively communicating with the inlet and outlet ports of the atleast one hydraulic pump and second ends opening to the outside of thecenter section through a pump case abutting surface of the centersection, and a first charging passage having a first end through whichworking hydraulic fluid is fed into the center section and a second endopening to the outside of the center section through the pump caseabutting surface of the center section. At least one of the pump caseand the center section forms a communication passage for communicationbetween the second ends of the pair of inlet/outlet passages and thesecond end of the first charging passage. A first hydraulic fluidfeeding valve is also provided in the pump unit for allowing hydraulicfluid to flow from the first charging passage to the pair ofinlet/outlet passages while preventing the reverse flow. The firsthydraulic fluid feeding valve is installable though the pump caseabutting surface of the center section or a center section abuttingsurface of the pump case.

With the pump unit of the above arrangement, the workload for boring thecenter section can be reduced, and assembly efficiency in assembling thepump unit can be improved.

The pump case of the pump unit of the first aspect of the presentinvention is preferably adapted to reserve the hydraulic fluid.

The pump unit of the first aspect of the present invention preferablyhas the following arrangement. Specifically, the working hydraulic fluidfed into the first charging passage through the first end thereof ishydraulic fluid fed from a charge pump operatively connected to adriving shaft for driving the at least one hydraulic pump. The centersection of the pump unit forms a second charging passage forcommunication between the inside of the pump case and the first chargingpassage; and the second charging passage preventing the flow of thehydraulic fluid from the first charging passage into the pump case,while allowing the hydraulic fluid stored within the pump case to flowinto the first charging passage when negative pressure is generated inat least one of the pair of inlet/outlet passages,

The communication passage of the pump unit of the first aspect of thepresent invention preferably has a groove shape and formed in the centersection abutting surface of the pump case. The pump case also has thecenter section abutting surface forming an escape groove communicatingwith the inside of the pump case for the escape of the leaked hydraulicfluid.

Further, the center section of the pump unit preferably forms a bypasspassage for communication between the pair of inlet/outlet passages; andthe bypass passage provided with an open/close valve in such a manner asto be operable from the outside for communication and cutoff of thehydraulic fluid between the pair of inlet/outlet passages.

According to the first aspect of the present invention, there is alsoprovided a pump unit that includes a first hydraulic pump with inlet andoutlet ports formed therein, a second hydraulic pump with inlet andoutlet ports formed therein disposed parallel to the first hydraulicpump, and a pump case for accommodating the first hydraulic pump and thesecond hydraulic pump. The pump case has an opening through which thefirst hydraulic pump and the second hydraulic pump are insertable intothe pump case. The center section is connected to the pump case in sucha manner as to close the opening of the pump case. The center sectionforms a first pair of inlet/outlet passages having first endsrespectively communicating with the inlet and outlet ports of the firsthydraulic pump and second ends opening to the outside of the centersection through a pump case abutting surface of the center section, asecond pair of inlet/outlet passages having first ends respectivelycommunicating with the inlet and outlet ports of the second hydraulicpump and second ends opening to the outside of the center sectionthrough the pump case abutting surface of the center section, and afirst charging passage having a first end through which workinghydraulic fluid is fed into the center section and a second end openingto the outside of the center section through the pump case abuttingsurface of the center section. At least one of the pump case and thecenter section forms a communication passage for communication thesecond ends of the first pair of inlet/outlet passages and the secondends of the pair of second inlet/outlet passages to the second end ofthe first charging passage. A first hydraulic fluid feeding valve isalso provided in the pump unit for allowing the flow of hydraulic fluidfrom the first charging passage to the pair of inlet/outlet passage andthe second pair of inlet/outlet passages. The first hydraulic fluidfeeding valve is installable through the pump case abutting surface ofthe center section or a center section abutting surface of the pumpcase.

According to the second aspect of the present invention, there isprovided a pump unit that includes a first hydraulic pump and a secondhydraulic pump respectively connected to first and second actuators viaa first pair of hydraulic lines and a second pair of hydraulic lines.The first hydraulic pump and the second hydraulic pump are accommodatedwithin a common housing with the former pump disposed parallel to thelatter pump. Both first and second hydraulic pumps are supported on acommon center section. The common center section forms a first pair ofinlet/outlet ports and a second pair of inlet/outlet ports. The firstpair of inlet/outlet ports respectively serve as connection ports forconnection with the first pair of inlet/outlet hydraulic lines, whilethe second pair of inlet/outlet ports respectively serve as connectionports for connection with the second pair of inlet/outlet hydrauliclines.

With the above arrangement, the conduit connection between the first andsecond actuators can be accomplished via the common center section,thereby achieving an improved efficiency in piping work. In addition,the first and second hydraulic pumps are accommodated within the commonhousing, so that the first and second hydraulic pumps can be installedon an object such as a vehicle through a single mounting operation.

The common center section of the pump unit of the second aspect of thepresent invention preferably forms a common charging passage for feedingpressurized hydraulic fluid into the first pair of hydraulic lines andthe second pair of hydraulic lines. With this arrangement, the number ofcharging lines can be reduced as compared with the arrangement where thefirst and second hydraulic pumps are separately installed, resulting ina lower manufacturing cost.

The first and second pair of inlet/outlet ports of the second aspect ofthe present invention are preferably formed in the same side of thecommon center section, thereby achieving an improved efficiency inpiping work between the first and second hydraulic motors.

The pump unit of the second aspect of the present invention preferablyhas the following arrangement. Specifically, the first hydraulic pumpand the second hydraulic pump respectively have pump shafts connectedtogether by a power transmission mechanism provided in the commonhousing. The common housing includes a partition wall through which thepump shafts of the first hydraulic pump and the second hydraulic pumpcan extend. The partition wall divides the common housing into a pumpaccommodation chamber and a power transmission mechanism accommodationchamber. With this arrangement, a single power transmission path issufficient for the simultaneous rotation of the pump shafts of the firstand second hydraulic pumps, resulting in a simplified structure of thepower transmission mechanism for the power transmission from the powersource to the pump unit. The partition wall can effectively preventforeign matters such as iron powder generated in the power transmissionmechanism from intruding into the pump accommodation chamber.

The pump unit of the second aspect of the present invention preferablyhas the following arrangement. Specifically, the first hydraulic pumpand the second hydraulic pump are of an axial piston type that includeangularly adjustable swash plates of a cradle type, respectively havingrear sides forming spherical convex surfaces. The partition wall formsguiding surfaces respectively sized and shaped to slidingly guide thespherical convex surfaces of the angularly adjustable swash plates alongthe guiding surfaces. These surface formations can achieve lowermanufacturing cost of the hydraulic pumps, and stabilized operation ofthe angularly adjustable swash plates.

According to the third aspect of the present invention, there isprovided a pump unit used for a vehicle with first and second hydraulicmotors respectively connected to the right and left drive wheels.Specifically, the pump unit includes a first hydraulic pump and a secondhydraulic pump, both being of a variable displacement type locatedparallel to one another within a common housing, and respectivelyoperable in association with the first and second hydraulic motors. Thefirst hydraulic pump and the second hydraulic pump respectively includea first pump shaft and a second pump shaft located parallel to oneanother within the common housing and operatively connected to oneanother via a power transmission mechanism. The first and second controlshafts are designed for controlling the input/output flow rates of thefirst and second hydraulic pumps. The first and second control shaftsrespectively extend away from one another along the vehicle widthdirection.

The pump unit of the above arrangement can achieve simplified mountingof the pump unit on the vehicle, and simplified structure for the powertransmission between the power source and the pump unit. When the pumpunit with the first and second control shafts extending away from oneanother along the vehicle width direction is mounted on a vehicle havingpush-pull control levers, the first and second control shafts can bedisposed parallel to the push-pull type control levers, therebyachieving a simplified link mechanism between the control shafts and thecontrol levers.

The first control shaft and the second control shaft of the third aspectof the present invention are preferably located substantially at thesame position with respect to the vehicle longitudinal direction. Thisarrangement can achieve a more simplified link mechanism between thecontrol shafts and the control levers.

The pump unit of the third aspect of the present invention preferablyhas the following arrangement. Specifically, the housing includes apartition wall between the first and second hydraulic pumps, and thepower transmission mechanism, through which the first pump shaft and thesecond pump shaft can extend. The partition wall divides the housinginto a hydraulic pump accommodation chamber and a power transmissionmechanism accommodation chamber, thereby effectively preventing anyforeign matters such as iron powder generated in the power transmissionmechanism from adversely affecting the pump performance.

The pump unit of the third aspect of the present invention preferablyhas the following arrangement. Specifically, the first and secondhydraulic pumps are of an axial piston type that respectively includeangularly adjustable swash plates of a cradle type respectively havingrear sides forming spherical convex surfaces. The partition wall formsguiding surfaces respectively sized and shaped to slidingly guide thespherical convex surfaces of the angularly adjustable swash plates. Withthis arrangement, the hydraulic pumps can be manufactured at low cost,and the angularly adjustable swash plates can be securely operated.

According to the fourth aspect of the present invention, there isprovided a pump unit for operation in association with first and secondactuators. The pump unit includes a first hydraulic pump and a secondhydraulic pump respectively connected to the first and second actuatorsvia a first pair of hydraulic lines and a second pair of hydrauliclines; a center section supporting the first hydraulic pump and thesecond hydraulic pump; a housing accommodating the first hydraulic pumpand the second hydraulic pump. The first hydraulic pump, the secondhydraulic pump, the first pair of hydraulic lines, the second pair ofhydraulic lines, the center section and the housing are integrallyconnected together to constitute a single unit. The pump unit alsoincludes a reservoir tank supportingly connected to the single unit forstoring hydraulic fluid to be replenished to the first pair of hydrauliclines and the second pair of hydraulic lines.

The pump unit of the above arrangement can improve an efficiency inmounting the first and second hydraulic pumps on an object such as avehicle, and shortening the length of the conduit for replenishing thehydraulic fluid from the reservoir tank to the first pair of hydrauliclines and the second pair of hydraulic lines, thereby lowering themanufacturing cost, and improving an efficiency in replenishing thehydraulic fluid through the decrease of the resistance force between thehydraulic fluid and the conduit wall, and producing other desirableeffects.

Preferably, the single unit of the pump unit of the fourth aspect of thepresent invention is designed so that the housing can serve as ahydraulic fluid tank, and the pump unit further includes a hydraulicfluid communication passage for providing a free fluid communicationbetween the reservoir tank and the housing. With this arrangement, thenumber of the conduits required between the first and second hydraulicpumps, and the first and second actuators can be reduced tosubstantially four conduits only, specifically the first pair ofhydraulic lines and the second pair of hydraulic lines. Thus, ascompared with the conventional arrangements, the pump unit of thisarrangement can achieve a lower manufacturing cost, an improvedassembling efficiency and an excellent workability in maintenance. Sincethe housing itself also serves as a hydraulic fluid tank, the reservoirtank can compactly be made.

The pump unit of the forth aspect of the present invention preferablyhas the following arrangement. Specifically, the center section of thepump unit is a unitary member for supporting both first and secondhydraulic pumps. The center section forms a first pair of hydraulicpassages respectively having first ends communicating with the firsthydraulic pump and second ends opening to the outside of the centersection to form connection ports for connection with the first pair ofhydraulic lines, a second pair of hydraulic passages respectively havingfirst ends communicating with the second hydraulic pump and second endsopening to the outside of the center section to form connection portsfor connection with the second pair of hydraulic lines, and a chargingpassage having a first end opening to the outside of the center sectionto form an inlet port for charging, serving as an inlet for thehydraulic fluid to be replenished and a second end communicating withthe first pair of hydraulic passages and the second pair of hydraulicpassages via check valves. The charging passage is connected to apressure relief line communicating with the housing via a relief vale,and the inlet port for charging is connected to the reservoir tank via ahydraulic fluid replenishing passage.

The pump unit of the fourth aspect of the present invention alsopreferably has the following arrangement. Specifically, the centersection includes a first center section and a second center sectionrespectively supporting the first hydraulic pump and the secondhydraulic pump. The first center section forms a first pair of hydraulicpassages respectively having first ends communicating with the firsthydraulic pump and second ends opening to the outside of the firstcenter section to form connection ports for connection with the firstpair of hydraulic lines. The second center section forms a second pairof hydraulic passages respectively having first ends communicating withthe second hydraulic pump and second ends opening to the outside of thesecond center section to form connection ports for connection with thesecond pair of hydraulic lines. At least one of the first and secondcenter sections forms a charging passage having a first end opening tothe outside of the at least one of the first and second center sectionsto form an inlet port for charging, serving as an inlet for thehydraulic fluid to be replenished, and a second end communicating withthe first pair of hydraulic passages and the second pair of hydraulicpassages via check valves. The charging passage is connected to apressure relief line communicating with the inside of the housing via arelief valve, and the inlet port for charging is connected to thereservoir tank via a hydraulic fluid replenishing passage.

The pump unit of the fourth aspect of the present invention alsopreferably has the following arrangement. Specifically, the pump unitincludes a cooling fan provided near the single unit. The cooling fan isadapted to be driven in synchronism with the first and second hydraulicpumps. The reservoir tank is connected to the single unit in such amanner as to form a clearance therebetween, into which a cooling airstream is drawn from the cooling fan. The hydraulic fluid communicationpassage and the hydraulic fluid replenishing passage are disposed insuch a manner to traverse the clearance. The thus arranged pump unit;can limit the temperature increase of the hydraulic fluid stored in thereservoir tank and the housing, and also effectively limit thetemperature increase of the hydraulic fluid flowing through thehydraulic fluid replenishing passage and the hydraulic fluidcommunication passage, thereby improving the transmission efficiencybetween the hydraulic pumps and the actuators.

According to the fifth aspect of the present invention, there isprovided a pump unit for operation in association with first and secondactuators. The pump unit includes: a first hydraulic pump and a secondhydraulic pump respectively connected to the first and second actuatorsvia a first pair of hydraulic lines and a second pair of hydrauliclines; a center section supporting the first hydraulic pump and thesecond hydraulic pump; and a housing accommodating the first hydraulicpump and the second hydraulic pump. The housing is adapted to be use asa hydraulic fluid tank. A hydraulic fluid circulation mechanism is alsoprovided for taking the hydraulic fluid from the hydraulic tank, andagain returning the same to the hydraulic tank. The hydraulic fluidcirculation mechanism is designed to cool the hydraulic fluid whilecirculating the same.

The pump unit of the above arrangement can effectively limit theincrease in temperature of the hydraulic fluid stored within thehydraulic tank, thereby effectively preventing deterioration in workingefficiency of a hydraulic actuation device.

Preferably, the circulation mechanism of the pump unit of the fifthaspect of the present invention includes a circulation line, at least aportion of which is formed by a conduit; the circulation line having afirst end communicating with the inside of the hydraulic tank and asecond end again communicating with the inside of the hydraulic tank,and the conduit has at least a portion provided thereon with coolingfins.

The pump unit of the fifth aspect of the present invention preferablyhas the following arrangement. Specifically, the center section is aunitary member on which the first and second hydraulic pumps aresupported in parallel relationship with one another. The housing formsan opening in a side thereof, through which the first and secondhydraulic pumps can pass. The center section and the housing areintegrally connected together to form a single unit, so that the openingof the housing can be sealed in a liquid tight manner by the centersection with the first and second hydraulic pumps supported thereon. Thecenter section forms a first pair of hydraulic passages respectivelyhaving first ends communicating with the first hydraulic pump and secondends opening to the outside of the center section to form connectionports for connection with the first pair of hydraulic lines, a secondpair of hydraulic passages respectively having first ends communicatingwith the second hydraulic pump and second ends opening to the outside ofthe center section to form connection ports for connection with thesecond pair of hydraulic lines, and a charging passage having a firstend communicating with the hydraulic fluid tank to form an inlet portfor charging, serving as an inlet for the hydraulic fluid to bereplenished and a second end communicating with the first pair ofhydraulic passages and the second pair of hydraulic passages via checkvalves. The pump unit further comprises: a charge pump for sucking thehydraulic fluid stored within the hydraulic fluid tank and thendischarging the same into the inlet port for charging; a pressure reliefline having a first end connected to the charging passage via a reliefvalve and a second end forming a drain port through which the hydraulicfluid from the relief valve is drained; and a pipe connecting the secondend of the pressure relief line with the hydraulic fluid tank; in whichthe pipe constitutes a conduit, and the charge pump constitutes a partof the hydraulic fluid circulation mechanism.

The pump unit of the fifth aspect of the present invention alsopreferably has the following arrangement. Specifically, the centersection includes a first center section and a second center sectionrespectively supporting the first and second hydraulic pumps. Thehousing has first and second sidewalls facing one another andrespectively forming a first opening and a second opening through whichthe first hydraulic pump and the second hydraulic pump can respectivelypass. The first and second center sections are integrally connected tothe housing to form a single unit, so that the first and second openingsof the housing ire sealed in a liquid tight manner by the first andsecond center sections respectively supporting the first and secondhydraulic pumps thereon. The first center section forms a first pair ofhydraulic passages respectively having first ends communicating with thefirst hydraulic pump and second ends opening to the outside of the firstcenter section to form connection ports for connection with the firstpair of hydraulic lines. The second section forms a second pair ofhydraulic passages respectively having first ends communicating with thesecond hydraulic pump and second ends opening to the outside of thesecond center section to form connection ports for connection with thesecond pair of hydraulic lines. At least one of the first and secondcenter sections forms a charging passage having a first endcommunicating with the hydraulic fluid tank to form an inlet for thehydraulic fluid to be replenished and a second end communicating withthe first pair of hydraulic passages and the second pair of hydraulicpassages via check valves. The pump unit further comprises: a chargepump for sucking the hydraulic fluid stored within the hydraulic fluidtank and then discharging the same into the inlet port for charging; apressure relief line having a first end connected to the chargingpassage via a relief valve and a second end forming a drain port throughwhich the hydraulic fluid from the relief valve is drained; and a pipeconnecting the second end of the pressure relief line with the hydraulicfluid tank; in which the pipe constitutes a conduit, and the charge pumpconstitutes a part of the hydraulic fluid circulation mechanism.

The pump unit of the fifth aspect of the present invention preferablyincludes a reservoir tank, in which the reservoir tank is in free fluidcommunication with the housing via a hydraulic fluid communicationpassage, and forms a hydraulic fluid tank in cooperation with thehousing, in which the inlet port for charging communicates with thereservoir tank via a hydraulic fluid replenishing passage.

The pump unit of the fifth aspect of the present invention preferablyincludes cooling fins provided on the hydraulic fluid replenishingpassage and the hydraulic fluid communication passage.

The pump unit of the fifth aspect of the present invention preferablyhas the following arrangement. Specifically, a cooling fan adapted to bedriven in synchronism with the first and second hydraulic pumps isprovided near the single unit. The reservoir tank is connected to thesingle unit in such a manner as to form a clearance therebetween, intowhich a cooling air stream from the cooling fan is drawn. The hydraulicfluid communication passage and the hydraulic fluid replenishing passageare disposed to transverse the clearance.

A cooling air duct is preferably provided in the pump unit of the fifthaspect of the present invention, so that a cooling air stream from thecooling fan is drawn into the clearance along the cooling air duct.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, and other objects, features and advantages of the presentinvention will become apparent from the detailed description thereof inconjunction with the accompanying drawings wherein.

FIG. 1 is an expansion plan view of a vehicle to which a pump unitaccording to first to fifth aspects of the present invention areapplied.

FIG. 2 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the first aspect of the presentinvention is applied.

FIG. 3 is a cross sectional plan view of the pump unit according to theembodiment of FIG. 2.

FIG. 4 is a perspective view with a partially exploded portion of thepump unit of FIGS. 2 and 3.

FIG. 5 is a cross section taken along lines V—V in FIG. 3.

FIG. 6 is a cross section taken along lines VI—VI in FIG. 3.

FIG. 7 is a cross section taken along lines VII—VII in FIG. 3.

FIG. 8 is a cross section taken along lines VIII—VIII in FIG. 3.

FIG. 9 is a cross section taken along lines IX—IX in FIG. 3.

FIG. 10 is a cross section taken along lines X—X in FIG. 3.

FIG. 11 is a cross section taken along lines XI—XI in FIG. 6.

FIG. 12 is a plan view of the pump unit of FIGS. 2 and 3.

FIG. 13 is a longitudinal cross section of a first center section ofanother embodiment of the pump unit according to the first aspect of thepresent invention.

FIG. 14 is a cross sectional plan view illustrating a modified exampleof a pump case in the pump unit of FIGS. 2 and 3.

FIG. 15 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the second aspect of the presentinvention is applied.

FIG. 16 is a longitudinal cross-sectional front view of the pump unitillustrated in FIG. 15.

FIG. 17 is a cross section taken along lines XVII—XVII in FIG. 16.

FIG. 18 is a cross section taken along lines XVIII—XVIII in FIG. 16.

FIG. 19 is a cross section taken along lines XIX—XIX in FIG. 16.

FIG. 20 is a longitudinal cross-sectional side view of the pump unitaccording to another embodiment of the second aspect of the presentinvention.

FIG. 21 is a longitudinal cross-sectional front view of the pump unitillustrated in FIG. 20.

FIG. 22 is a cross section taken along lines XXII—XXII in FIG. 20.

FIG. 23 is a cross section taken along lines XXII—XXII in FIG. 20.

FIG. 24 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the third aspect of the presentinvention is applied.

FIG. 25 is a longitudinal cross-sectional front view of the pump unitillustrated in FIG. 24.

FIG. 26 is a cross section taken along lines XXVI—XXVI in FIG. 25.

FIG. 27 is a cross section taken along lines XXVII—XXVII in FIG. 25.

FIG. 28 is a cross section taken along lines XXVIII—XXVIII in FIG. 25.

FIG. 29 is a longitudinal cross-section al side view of the pump unitaccording to another embodiment of the third aspect of the presentinvention.

FIG. 30 is a longitudinal cross-sectional front view of the pump unitillustrated in FIG. 29.

FIG. 31 is a cross section taken along lines XXXI—XXXI in FIG. 29.

FIG. 32 is a cross section taken along lines XXXII—XXXII in FIG. 29.

FIG. 33 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the fourth aspect of the presentinvention is applied.

FIG. 34 is a longitudinal cross-sectional side view of the pump unitillustrated in FIG. 33.

FIG. 35 is a cross section taken along lines XXXV—XXXV in FIG. 34.

FIG. 36 is a cross section taken along lines XXXVI—XXXVI in FIG. 34.

FIG. 37 is an enlarged view of a portion XXXVII in FIG. 34.

FIG. 38 is a cross section taken along lines XXXVIII—XXXVIII in FIG. 37.

FIG. 39 is a cross section taken along lines XXXIX—XXXIX in FIG. 37.

FIG. 40 is a cross section taken along lines XXXX—XXXX in FIG. 39.

FIG. 41 is a longitudinal cross-sectional side view of the pump unitaccording to another embodiment of the fourth aspect of the presentinvention.

FIG. 42 is a cross section taken along lines XXXXII—XXXXII in FIG. 41.

FIG. 43 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the fifth aspect of the presentinvention is applied.

FIG. 44 is a longitudinal cross-sectional side view of the pump unitillustrated in FIG. 43.

FIG. 45 is a cross section taken along lines XXXXV—XXXXV in FIG. 44.

FIG. 46 is a cross section taken along lines XXXXVI—XXXXVI in FIG. 44.

FIG. 47 is a cross section taken along lines XXXXVII—XXXXVII in FIG. 44.

FIG. 48 is a cross section taken along lines XXXXVIII—XXXXVIII in FIG.44.

FIG. 49 is a cross section taken along lines XXXXIX—XXXXIX in FIG. 47.

FIG. 50 is a perspective view of a mounting member.

FIG. 51 is a longitudinal cross-sectional side view of the pump unitaccording to another embodiment of the fifth aspect of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment

The first embodiment of the pump unit according to the first aspect ofthe present invention will be hereinafter described with reference tothe accompanying drawings.

A pump unit 100 according to the first aspect of the present inventionis designed to be operated in association with an actuator that isconnected thereto via first and second pairs of hydraulic lines 184 aand 184 b and driven through an effect of pressurized hydraulic fluid inthe pair of hydraulic lines. This embodiment will be described by takingfor example the case that hydraulic motors 182 a and 182 b each are usedas the actuator.

FIGS. 1 and 2 are respectively an expansion plan view of a vehicle towhich the pump unit 100 of this embodiment is applied, and a hydrauliccircuit of the vehicle. FIG. 3 is a cross sectional plan view of thepump unit and its periphery. FIG. 4 is a perspective view with apartially exploded portion of the pump unit. FIGS. 5 to 10 arerespectively cross sections taken along lines V—V, VI—VI, VII—VII,VIII—VIII, IX—IX, and X—X in FIG. 3. FIG. 11 is a cross section takenalong lines XI—XI in FIG. 6. The reference codes 185, 197 a and 197 b,and 199 in FIG. 1 respectively represent a reservoir tank, casterwheels, and a driver seat.

As illustrated in FIGS. 2 and 3, the pump unit 100 includes a firsthydraulic pump 110 a, a second hydraulic pump 110 b, a pump case 120that accommodates the first and second hydraulic pumps 110 a and 110 band has an opening 121 c through which the hydraulic pumps are insertedinto the pump case 120, and a lid or center section 130 connected to thepump case so as to close the opening of the pump case.

In this embodiment, the pump unit 100 is explained as having a pair ofhydraulic pumps. However, the first aspect of the present invention isnot necessarily limited to this arrangement. Rather, it is applicable tothe case where a single hydraulic pump is used, or three or more thanthree hydraulic pumps are used.

As illustrated in FIG. 2, the first and second hydraulic pumps 110 a and110 b are of a variable displacement type, which has a variableinput/output flow rates through the operation of a swash plate. Thehydraulic pumps 110 a and 110 b are respectively connected to the firstand second hydraulic motors 182 a and 182 b serving as the actuator, viathe first pair of hydraulic lines 184 a and the second pair of hydrauliclines 184 b.

Accordingly, varying the input/output flow rates of each of thehydraulic pumps 110 a and 11 b through the swash plate causes thepressure difference between the first pair of hydraulic lines 184 a, andthe second pair of hydraulic lines 184 b. According to the operationalangle of the swash plate, motor shafts of the first hydraulic motor 182a and/or the second hydraulic motor 182 b are rotated, and drive wheels183 a and 183 b that are operatively connected to the motor shafts aredriven. The reference codes 180 and 181 in FIG. 2 respectively representa power source and a cooling fan.

As described above, the first and second hydraulic pumps 110 a and 110 baccording to this embodiment are of the variable displacement type, andthe first and second hydraulic motors 182 a and 182 b in associationwith the hydraulic pumps 110 a and 110 b are of a fixed displacementtype. However, the first aspect of the present invention is notnecessarily limited to this arrangement. That is, it is possible toemploy the hydraulic pumps of the fixed displacement type, and thehydraulic motors of the variable displacement type driven by thehydraulic pumps or the hydraulic pumps and the hydraulic motors, both ofwhich are of the variable displacement type.

In this embodiment, the first and second hydraulic pumps 110 a and 110 bare of an axial piston type. Alternatively, the pump unit may employ thehydraulic pumps of a radial piston type.

As illustrated in FIGS. 3 and 5, the first and second hydraulic pumps110 a and 110 b respectively include a first hydraulic pump shaft 111 aand a second hydraulic pump shaft 111 b, both of which are disposedparallel to one another, a first piston unit 112 a and a second pistonunit 112 b that are reciprocatingly movable according to the rotation ofthe pump shafts, a first cylinder block 113 a and a second cylinderblock 113 b that reciprocably support the piston units, a firstangularly adjustable swash plate 114 a and a second angularly adjustableswash plate 114 b that regulate the stroke lengths of the piston unitsby varying their tilting angles to vary their input/output flow rates,and a first control shaft 115 a and a second control shaft 115 b thatcontrol the tilting angles of these swash plates.

As illustrated in FIG. 5, the first control shaft 115 a has an inner endextending into the pump case 120 and connected to the first angularlyadjustable swash plate 114 a via an arm 116 a, and an outer endextending vertically above the pump case 120. The second control shaft115 b has a similar arrangement (not shown).

In this embodiment, the pump unit 100 is of a horizontal type that hasthe horizontally extending first and second hydraulic pump shafts 111 aand 111 b. However, the first aspect of the present invention is notnecessarily limited to this arrangement. Rather, it is a matter ofcourse to employ the pump unit of a vertical type that has thevertically extending first and second hydraulic pump shafts 111 a and111 b.

The pump unit 100 further includes a neutral position return mechanism150 that returns the swash plates 114 a and 114 b of the first andsecond hydraulic pumps 110 a and 110 b to their respective neutralpositions. A plan view of a portion of the pump unit 100 is illustratedin FIG. 12.

The neutral position return mechanism 150 includes a first neutralposition return mechanism 150 a for the first hydraulic pump 110 a, anda second neutral position return mechanism 150 b for the secondhydraulic pump 110 b disposed on a common base plate 150 c mounted onthe upper surface of the pump case 120. The following description, whichwill be made for the first neutral position return mechanism 150 a, willalso be applicable for the second neutral position return mechanism 150b.

As illustrated in FIG. 12, the first neutral position return mechanism150 a includes a connecting arm 151 with a first end pivotably connectedto a connecting member 192 a connected to a control lever 198 a (seeFIG. 1) and a second end non-rotatably connected to the control shaft115 a, a swing arm 152 a with a proximal end non-rotatably connected tothe control shaft 115 a and a distal end as a free end, and an eccentricpin 153 a detachably fixed to the upper surface of the pump case 120.

The swing arm 152 a has a distal edge defining a deepest part 193 a nearthe shaft center of the control shaft 115 a, and cam surfaces laterallyextending from the deepest part in such a manner as to graduallyincrease the distance from the shaft center of the control shaft 115 aas they extend away from the deepest part.

The eccentric pin 153 a has a first shaft portion 195 a mounted on theupper surface of the base plate 150 c, and a second shaft portion 196 aextending upwardly from the first shaft portion 195 a. The second shaftportion has the shaft center eccentric to that of the first shaftportion, so that the second shaft portion 196 a has the shaft centerrotated around the shaft center of the first shaft portion 195 a throughthe rotation of the first shaft portion 195 a around the shaft centerthereof.

The first neutral position return mechanism 150 a further includes apressing arm 154 a with a proximal end rotatably supported on the secondshaft portion 196 a of the eccentric pin 150, and a distal end as a freeend. The pressing arm 154 a is provided with a roller 155 a engageablewith the distal edge of the swing arm 152 a. The pressing arm 154 a andthe swing arm 152 a are disposed in such a manner as to have the roller155 a engageable with the deepest part 193 a of the swing arm 152 a,when the swash plate of the hydraulic pump 110 a lies at the neutralposition.

Specifically, the pressing arm 154 a is positioned such that the roller155 a is engaged with the deepest part 193 a of the swing arm 152 a,while holding the swash plate of the hydraulic pump 110 a at a positionto be considered as the neutral position. At this time, there may occurthe case where the hydraulic pump 110 a is not brought into a neutralstate due to assembling errors or the like. Specifically, there mayoccur the case the swing arm 152 a must be rotated by a predeterminedangle in either direction around the shaft center of the control shaft115 a from a predetermined design position illustrated in FIG. 12, inorder to have the swash plate lying at the neutral position.

Even if such a positional error occurs, the first neutral positionreturn mechanism 150 can adjust the relative displacement between thecontrol shaft 115 a and the connecting member 192 a easily.Specifically, in the first neutral position return mechanism 150, theeccentric pin 153 a has the second shalt portion 196 a as the supportingpoint of the pressing arm 154 a, which shaft portion is eccentric to thefirst shaft portion 195 a, thereby allowing the second shaft portion 196a to have the shaft center easily adjustable through the rotation of thefirst shaft portion 195 a around the shaft center thereof, and hence theroller 155 a to have an easily adjustable distance relative to the swingarm 152 a. Thus, the roller 155 a can easily be brought into engagementwith the deepest part 193 a of the swing arm 152 a, even when thehydraulic pump 110 a cannot come into the neutral state without therotation of the swing arm 152 a by a predetermined angle around theshaft center of the control shaft 115 from the predetermined designposition.

The first neutral position return mechanism 150 a further includes aspring member 156 a to urge the roller 155 a towards the distal edge ofthe swing arm 152 a.

The first neutral position return mechanism 150 a having the abovearrangement performs in the following manner. When the driver operatesthe mechanism 150 a through the shifting operation of a control lever198 a provided near the driver seat, the connecting member 192 a isslidingly moved along either direction (F or R with N therebetween)indicated by the arrows of FIG. 12 according to the shifting operationof the control lever 198 a, thereby pivotally moving the connecting arm151 a, and hence rotating the control shaft 115 a. The swash plate canbe thus tilted according to the shifting operation of the control lever.

On the other hand, when the driver releases the control lever 198 a fromits operational state where the swash plate is held in a tiltedposition, the first neutral position return mechanism 150 a canautomatically return the swash plate of the hydraulic pump to theneutral position. Specifically, since the swash plate tilted in eitherdirection from the neutral position causes the control shaft 115 a to berotated around the shaft center thereof in either direction from theneutral position, the swing arm 152 a is pivotally moved in eitherdirection around the shaft center of the control shaft 115 a accordingto the rotation angle of the control shaft 115 a. Accordingly, theroller 155 a is engaged with one of the cam surfaces defined by thedistal edge of the swing arm 152 a. As described above, since the roller155 a is constantly urged towards the distal edge of the swing arm 152 aby the spring member 156 a, the swing arm 152 a automatically returns tothe neutral position, at which the roller 155 a is engaged with thedeepest part 193 a, through a camming effect between the roller 155 aand the cum surface 194 a by releasing the control lever from theoperational state where the roller 155 a is engaged with the cum surface194 a.

Thus, the first neutral position return mechanism 150 a performs so thatthe swash plate of the hydraulic pump 110 a automatically returns to theneutral position in response to releasing the control lever 198 a fromthe operational state.

The second neutral position return mechanism 150 b has a similararrangement. Accordingly, the right and left control levers 198 a and198 b are controlled independently of each other, so that the first andsecond hydraulic pumps can have the swash plates tilted independently ofeach other. Both control levers 198 a and 198 b are released from theoperational states to stop the vehicle without any delay.

As best illustrated in FIG. 3, the pump unit 100 further includes apower transmission mechanism 158 that is accommodated within the housing120 to operatively connect the first hydraulic pump shaft 111 a to thesecond hydraulic pump shaft 111 b.

The power transmission mechanism 158 provided in the pump unit 100 cansimultaneously drive both hydraulic pump shafts 111 a and 111 b only byconnecting the power source 180 to either one of the first hydraulicpump shaft 111 a and the second hydraulic pump shaft 111 b, resulting ina simple transmission arrangement between the power source 180 and thepump unit 100.

In this embodiment, the power transmission mechanism 158 is in the formof a gear transmission device that includes a first gear 158 anon-rotatably supported on the first hydraulic pump shaft 111 a, and asecond gear 158 b non-rotatably supported on the second hydraulic pumpshaft 111 b in meshed engagement with the first gear 158 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

The pump case 120 is sealed in a liquid tight manner by the centersection 130, thereby reserving the hydraulic fluid within the pump case120. Specifically, the pump case 120 also serves as a part of areservoir tank. The reference code 145 in FIGS. 4 and 7 represents ahole used along with a separately provided hydraulic fluid tank tocommunicate the inside of the pump case with the hydraulic fluid tank.

As illustrated in FIG. 3, the pump case 120 includes a first pump case121 for accommodating the first and second hydraulic pumps 110 a and 110b, and a second pump case 122 for accommodating the power transmissionmechanism 158.

As best illustrated in FIGS. 3 and 5, the first pump case 121 has a boxshape with a first side wall 121 a disposed in either side along thelongitudinal direction of the hydraulic pump shafts 111 a and 111 b, orin this embodiment in the front side of the vehicle, which will behereinafter referred to simply as the front side, and a peripheral wall121 b extending from a peripheral edge of the first sidewall 121 a tothe opposite side of the pump unit 100 along the longitudinal directionof the hydraulic pump shafts 111 a and 111 b (i.e., the rear side of thevehicle in this embodiment, which will be referred to simply as the rearside). The first sidewall 121 a forms bearing holes through which thefirst and second hydraulic pump shafts 111 a and 111 b respectivelyextend. The rear side has an end surface defining the opening 121 c forreceiving the first and second hydraulic pumps 110 a and 110 b. Theopening of the pump case 121 is sealed by the center section 130 in aliquid tight manner.

The second pump case 122 has a box shape with a front sidewall 122 a anda peripheral wall 122 b extending from a peripheral edge of the frontsidewall 122 a to the rear side to form a box shape. The front sidewall122 a forms a bearing hole through which the front end portion of thefirst hydraulic pump shaft 111 b extend, and a bearing portion forsupporting the front end portion of the second hydraulic pump shaft 111b. The rear side of the second pump case 122 has an end surface formingan opening 122 c for receiving the power transmission mechanism 150.

The second pump case 122 is connected to the first pump case 121 so thatthe opening 122 c can be sealed by the first sidewall 121 a of the firstpump case 121 in a liquid tight manner, and forms an accommodation spaceof the power transmission mechanism 158 in cooperation with the firstsidewall 121 a of the first pump case 121.

In the thus arranged pump case 120, the first sidewall 121 a of thefirst pump case 121 serves as a partition wall dividing the pump caseaccommodation space into a hydraulic pump accommodation chamber and apower transmission mechanism accommodation chamber. The partition wallthus defining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 158 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 112 a, 112 b, cylinder blocks113 a, 113 b and/or other parts. A seal ring, oil seal or the like mayalso be provided around the peripheral surface of the first and secondhydraulic pump shafts 111 a and 111 b extending through the partitionwall 121 a to more securely prevent the intrusion of the foreignmatters.

Portions of the pump case 120, through which the respective shafts 111a, 115 a and 115 b extend, are sealed by any suitable sealing means in aliquid tight manner, thereby allowing the pump case 120 to serve as ahydraulic fluid tank.

As illustrated in FIG. 6, the first sidewall 121 a serving as thepartition wall more preferably forms a hydraulic fluid communicationhole 123 for communication between the hydraulic pump accommodationchamber and the power transmission mechanism accommodation chamber witha filter 124 provided in the hole for preventing the intrusion of theforeign matters into the hydraulic pump accommodation chamber. The thusformed hydraulic fluid communication hole 123 can omit the necessity ofseparately feeding lubricant to the power transmission mechanism 158,with the result that the power transmission mechanism 158 can belubricated with the hydraulic fluid stored within the pump case 120.This permits low manufacturing cost and ease of maintenance.

Hydraulic fluid communication holes 123 are preferably and respectivelybe provided with the meshed point of the first gear 158 a and the secondgear 158 b therebetween, and more particularly at the downstream andupstream sidles with respect to the rotational direction of the firstgear 158 a and the second gear 158 b. The thus arranged hydraulic fluidcommunication holes 123 achieve an efficient circulation of thehydraulic fluid between the hydraulic pump accommodation chamber and thepower transmission mechanism accommodation chamber.

In this embodiment, the first and second angularly adjustable swashplates 114 a and 114 b are of a cradle type, as illustrated in FIG. 3.Therefore, when the partition wall 121 a forms, on its side facing thehydraulic pumps 110 a, 110 b, spherical concave surfaces 126 a and 126 brespectively adapted to spherical convex surfaces 116 a and 116 b formedin the rear portions of the swash plates 114 a and 114 b, which rearportions being opposite to the surfaces facing the piston units 112 aand 112 b, the spherical concave surfaces 126 a and 126 b can slidinglyguide the spherical convex surfaces 116 a and 116 b of the swash plates114 a and 114 b thereon. The swash plates thus can be securelypositioned on the spherical concave surfaces 126 a and 126 b.

In this embodiment, the first sidewall 121 a of the first pump case 121serves as the partition wall. Alternatively, a partitioning means maytake various forms, as long as it can produce the same effect. Forexample, a separately prepared partition wall 121 a′ may be mounted in afirst pump case 121′ having a simple box shape, as illustrated in FIG.14. This arrangement is advantageous in that the spherical concavesurfaces 116 a and 116 b can easily be formed.

Now, the description will be made for the center section 130. As bestillustrated in FIGS. 3 and 5, the center section 130 includes a firstcenter section 131 connected to the pump case 120 through direct contactto the rear side of the pump case 120 for sealing the opening 121, and asecond center section 132 connected to the first center section 131 insuch a manner as to surround a charge pump 160 operatively driven by thehydraulic pump shaft 111 a of the first hydraulic pump 110 a.

The center section 130, as illustrated in FIGS. 4, 5 and 7, forms afirst pair of inlet/outlet passages 133 a having first ends respectivelycommunicating with inlet and outlet ports of the first hydraulic pump110 a and second ends opening to the outside of the center section 130through a pump-case abutting surface 131 a of the first center section131, and a second pair of inlet/outlet passages 133 b having first endsrespectively communicating with inlet and outlet ports of the secondhydraulic pump 110 b and second ends opening to the outside of thecenter section 130 through the pump-case abutting surface 131 a of thefirst center section 131. Both pairs of the passages 133 a and 133 b aredisposed parallel with each other.

The center section 130 forms a first charging passage 134 having a firstend communicating with an outlet port of the charge pump 160, and asecond end formed in the pump-case abutting surface 131 a of the firstcenter section 131.

The first pair of inlet/outlet passages 133 a constitutes a part of thefirst pair of hydraulic lines 184 a communicating between the firsthydraulic pump 110 a and the first hydraulic motor 183 a. On the otherhand, the second pair of inlet/outlet passages 133 b constitutes a partof the second pair of hydraulic lines 184 b communicating between thesecond hydraulic pump 110 b and the second hydraulic motor 183 b (seeFIG. 2).

As illustrated in FIG. 2, the center section 130 forms a pressure reliefline 135 communicating with the first charging passage 134. The pressurerelief line 135 is provided therein with a relief valve 161 foradjusting the hydraulic pressure of the first charging passage 134. Inthis embodiment, the relief valve 161 is disposed in the second centersection 132, as illustrated in FIGS. 10 and 11.

At least one of the pump case 120 and the center section 130 forms acommunication passage communicating between the first pair ofinlet/outlet passages 133 a and the second pair of inlet/outlet passages133 b, and the first charging passage 134 via their second ends.

In this embodiment, a first center section abutting surface 121 e of thefirst housing, as best illustrated in FIGS. 4 and 8, forms a singlefluid groove 136 extending over the second ends of the first pair ofinlet/outlet passages 133 a and the second pair of inlet/outlet passages133 b. The first charging passage 134 has the second end communicatingwith the fluid passage groove 136.

Charging check valves 162 a, 162 b, 162 c and 162 d are provided in sucha manner as to be installable through the abutting surface 131 a of thefirst center section 131 or the abutting surface 121 e of the firsthousing 121, as best illustrated in FIG. 4. These valves are designed toallow the flow of the hydraulic fluid from the first charging passage134 to the first pair of inlet/outlet passages 133 a and the second pairof inlet/outlet passages 133 b, while preventing the reverse flow.

The installation of those charging check valves through the first centersection abutting surface 121 e of the first housing 121 or the firsthousing abutting surface of the first center section 131 produces thefollowing effects.

Specifically, since a die pattern of the fluid passage groove 136 can beformed in a die for the first housing 121 or the first center section121, it is not necessary to additionally bore holes for receiving thecharging check valves 162 a to 162 d. This omits the necessity of aconventionally required machining process, resulting in a lowermanufacturing cost.

Since the charging check valves 162 can be secured in position only byconnecting the first housing 121 to the first center section 131, acovering member or other check valve fixing means can be omitted. Thisarrangement can reduce the number of parts, resulting in a lowermanufacturing cost and an improved assembling efficiency.

As illustrated in FIGS. 4, 8 and 11, the center section abutting surface121 e of the pump case 120 forms an escape groove 137 surrounding thefluid passage groove 136 and having at least one terminal portioncommunicating with the inside of the pump case for the discharge of theleaked hydraulic fluid.

With the escape groove 137, the hydraulic fluid, which flows from thefirst charging passage 134 to the first pair of inlet/outlet passages133 a and the second pair of inlet/outlet passages 133 b via the fluidpassage groove 136, is prevented from leaking to the outside through theabutting portion between the first pump case 121 and the first centersection 131. Specifically, the hydraulic fluid leaked out of the fluidpassage groove 136 is held in the escape groove 137, and then returnedto the inside of the pump case 120. Whereby, the leakage of thehydraulic fluid outwards of the case from the abutting portion betweenthe first pump case 121 and the first center section 131 can effectivelybe prevented.

Leak lines 163 a and 163 b each having a throttle valve are preferablyformed between the first charging passage 134 and at least one of thefirst pair of inlet/outlet passages 133 a, and between the firstcharging passage 134 and at least one of the second pair of inlet/outletpassages 133 b (see FIG. 2).

The leak lines 163 a and 163 b are designed to assure the neutralizationof the hydraulic pumps 110 a and 110 b. Specifically, even if the swashplates 114 a and 114 b of the hydraulic pumps 110 a and 110 b tilt fromthe neutral positions by a small angle, there occurs the pressuredifference between the first pair of hydraulic lines 184 a, and/orbetween the second pair of hydraulic lines 184 b. This pressuredifference causes the rotation of the hydraulic motors 182 a and 182 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 114 a and 114 b due to assembling error or the like causes anunintentional rotation of the hydraulic motors 182 a and 182 b. On thecontrary, the leak lines 163 a and 163 b, as described above, allow thepressurized hydraulic fluid to leak therethrough from the first pair ofhydraulic lines 184 a or the second pair of hydraulic lines 184 b. Thus,the swash plates can have the neutral positions of a broadened effectivearea by effectively limiting the pressure difference between the pair offirst hydraulic lines 184 a, and/or between the second pair of hydrauliclines 184 b, thereby effectively avoiding the unintentional rotation ofthe hydraulic motors 182 a and 182 b, even for the swash plates 114 aand 114 b having the actual neutral position displaced from the designneutral position due to the assembling errors or the like.

In view of transmission efficiency between the hydraulic pumps 110 a,110 b and the hydraulic motors 182 a, 182 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 184 a, 184 b through the leak lines 163 a, 163 b is notpreferable. Therefore, the leak lines 163 a, 163 b are preferablyprovided in portions from the first charging passage 133 to one of thefirst pair of inlet/outlet passages 133 a, and to one of the second pairof inlet/outlet passages 133 b, and more preferably to one of the firstpair of inlet/outlet passages 133 a which has a higher pressure duringrearward movement of the vehicle. This is because the forward movementof the vehicle frequently occurs as compared with the rearward movement.

The first center section 131, as illustrated in FIGS. 2 and 7, forms afirst bypass passage 138 a for communication between the first pair ofinlet/outlet passages 133 a, and a second bypass passage 138 b forcommunication between the second pair of inlet/outlet passages 133 b.Although the following description will be made for the first bypasspassage 138 a, it is also applicable for the second bypass passage 138b.

In this embodiment, the first pair of inlet/outlet passages 133 a areformed parallel to one another, and the first bypass passage 138 a isformed orthogonal to the pair of first inlet/outlet passages, as bestillustrated in FIG. 7. This arrangement achieves the communicationbetween the first pair of inlet/outlet passages 133 a by forming only asingle hole.

The first bypass passage 138 a includes a first bypass valve 140 aadapted to take a communication position and a cutoff positionrespectively for bringing the first pair of inlet/outlet passages 133 ainto and out of communication with one another. The first bypass valve140, as illustrated in FIG. 7, has a proximal end portion 141 aextending to the outside of the first center section 131 to be operatedfrom the outside of the first center section 131, allowing the firstbypass valve 140 a to take the communication position and the cutoffposition.

Specifically, the first bypass passage 138 a includes an inwardlythreaded portion 139 a having a proximal end opening to the outside ofthe first center section 131 and an inner threaded circumferentialsurface, a middle portion 139 b extending inwardly from the inner end ofthe inwardly threaded portion 139 a in such a manner as to straddle anadjacent one of the first pair of inlet/outlet passages 133 a, a distalend portion 139 c having a diameter smaller than the middle portion 139b with a stepped portion and communicating with a remote one of thefirst pair of inlet/outlet passages 133 a.

On the other hand, the first bypass valve 140 a includes a proximal endportion 141 a lying outside of the first center section 131, anoutwardly threaded portion 141 b distally extending from the proximalend portion 141 a and having an outer threaded circumferential surfacefor threaded engagement with the inner threaded circumferential surface139 a, a seal portion 141 c distally extending from the outwardlythreaded portion 141 b and liquid-tightly engageable with the middleportion 139 b at the proximal side with respect to the adjacent one ofthe first pair of inlet/outlet passages 133 a, and an abutting portion141 d distally extending from the seal portion 141 c and having a shapeadapted to the stepped portion for sealed contact between the abuttingportion and the stepped portion. The first bypass valve 140 a thus cantake the cutoff position with the abutting portion 141 d abutting thestepped portion, and the communication position with the abuttingportion 141 d located away from the stepped portion, through the axialsliding motion of the first bypass valve 140 a caused by the rotation ofthe first bypass valve 140 a around the axis thereof via the proximalend portion 141 a.

A release means including the first bypass passage 138 a and the firstbypass valve 140 a, as well as the second bypass passage 138 b and thesecond bypass valve 140 b is designed to easily move the vehicle, whenthe vehicle must forcibly be moved or the vehicle wheels must forciblybe rotated by man power or the like due to the disorder of the powersource 180, the hydraulic pumps 110 a, 110 b or the like. Specifically,when the vehicle wheels connected to the hydraulic motors 182 a and 182b are forcibly rotated with the first pair of hydraulic lines 184 aand/or the second pair of hydraulic lines 184 b lying in the closingstate, there occurs the pressure difference between the first pair ofhydraulic lines 184 a, and between the second pair of hydraulic lines184 b. As a result, the vehicle is hardly moved, or the vehicle wheelsare hardly rotated. On the contrary, the release means can easilyachieve the communications between the first pair of hydraulic lines 184a, and between the second pair of hydraulic lines 184 b without thenecessity of mechanically releasing all the check valves 162 a to 162 d.Whereby, the vehicle can easily be moved by man power or the like.

In this embodiment, the first pair of inlet/outlet passages 133 a andthe second pair of inlet/outlet passages 133 b, as illustrated in FIG.7, respectively have connecting ports formed in the same side of thefirst center section 31, resulting in an easy piping work between theseconnecting ports and the hydraulic motors 182 a and 182 b.

As illustrated in FIGS. 2 and 5, the first center section 131 and thesecond center section 132 forms a second charging passage 142 with afirst end communication with the inside of the pump case 120 and asecond end communicating with the first charging passage 134. The secondcharging passage 142 is designed to prevent the flow of the hydraulicfluid from the first charging passage 134 to the pump case 120, and feedan additional amount of the hydraulic fluid from the pump case 120 tothe first and second pairs of hydraulic lines 184 a and 184 b when theselines have a reduced amount of the hydraulic fluid.

In this embodiment, the second charging passage 142 includes a checkvalve 143 for allowing the flow of the hydraulic fluid from the pumpcase 120 to the first charging passage 134 while preventing the reverseflow, thereby producing the above mentioned effect. Although the chargepump 160 may be some what deteriorated in operation efficiently, it ispossible to employ a throttle valve instead of the check valve 143.

Providing the second charging passage 142 can effectively prevent thevehicle from being brought into a so-called free wheel phenomenon, whichoccurs when the vehicle stopping on a sloping road is accidentally moveddownwardly, causing the rotation of the wheels. That is, for the stoppedvehicle, the hydraulic pumps 110 a and 110 b have the swash plates lyingat the neutral position. In this state, when the vehicle is stopped on,for example, a slopped road, the vehicle is subjected to a forcepossibly causing the rotation of the wheels through the vehicle weight,or the rotation of the motor shafts of the hydraulic motors 182 a and182 b. Since the hydraulic pumps 110 a and 110 b are set in the neutralstate, the hydraulic pumps 182 a and 182 b subjected to such forcecauses one of the first pair of hydraulic lines 184 a and one of thesecond pair of hydraulic lines 184 b to have a highly pressurizedhydraulic fluid, and the remaining ones to have a low pressurizedhydraulic fluid. The hydraulic fluid exceeding a predetermined pressurelevel in the hydraulic lines causes the leakage through a gap incylinder blocks of the hydraulic pumps in communication with such highlypressurized hydraulic lines. Whereby, the amounts of the hydraulicfluids in the pairs of hydraulic lines are reduced, facilitating thefree rotation of the motor shafts easier.

On the contrary, the second charging passage 142 of this embodimentsucks the hydraulic fluid within the pump case when the first pair ofhydraulic lines 184 a has reduced amount of the hydraulic fluid,resulting in a negative pressure in these lines. Specifically, thesecond charging passage 142 can prevent the pair of the hydraulic linesfrom having a reduced amount of the hydraulic fluid, and hence the motorshafts from being freely rotated.

As illustrated in FIGS. 3, 6, 9 and 10, the second center section 132 isprovided at its rear end with a filter 186. The second center section132 forms an inlet line 165 having a first end communication with thecharge pump 160 a through its inlet opening and a second endcommunicating with the filter 186, and a filter line 166 having a firstend communicating with the filter 186 and a second end communicatingwith a hydraulic fluid tank (not shown), thereby allowing the hydraulicfluid fed from the tank and passing through the filter 186 and thefilter line 166 to be sucked into the charge pump 160 through the inletopening.

The pump unit 100 of this embodiment constitutes a single unit byunitedly connecting the first and second hydraulic pumps 110 a and 110b, t he center section 130 and the housing 120 together. Therefore, bothfirst and second pumps 110 a and 110 b can be installed on the vehicleonly by mounting the single unit on the vehicle, resulting in animproved efficiency in assembling the vehicle.

Second Embodiment

The second embodiment of the first aspect of the present invention willbe described with reference to FIG. 13. FIG. 13 is a longitudinal crosssection of a first center section 131′ of a pump unit 100 according tothis embodiment, which figure corresponding to FIG. 7 illustrating theaforementioned first embodiment.

In this embodiment, the first bypass line 138 a and the second bypassline 138 b are replaced by a single common line 138. In the followingdescription, corresponding or identical parts to those of the firstembodiment have been given the same reference characters or those withprimes (′) to omit a detailed description thereof.

The common bypass line 138′ has a proximal end portion opening outwardlyand a distal end portion communicating with all the first and secondpairs of inlet/outlet passages 133 a and 133 b.

The common bypass line 138′ includes a single bypass valve 140′ to beoperated from the outside of the first center section 131′ for thecommunication and the cutoff of the hydraulic fluid between the firstpair of inlet/outlet passages 133 a, and between the second pair ofinlet/outlet passages 133 b.

In addition to the desirable effects produced by the first embodiment,the pump unit of the second embodiment can achieve an effective boringoperation and lower manufacturing cost through the reduction of thenumber of parts.

In each of the aforementioned embodiments, the description has been madefor the case that a pair of hydraulic pumps is included. However, it isnot necessary to limit the number of the hydraulic pumps to that ofthese embodiments. The present invention is applicable to thearrangement where a single hydraulic pump, or more than two hydraulicpumps are included.

Third Embodiment

One embodiment of the pump unit according to the second aspect of thepresent invention will be hereinafter described with reference to FIGS.15 to 19. FIG. 15 is a hydraulic circuit diagram of the vehicle to whicha pump unit 200 of this embodiment is applied. FIG. 16 is a longitudinalcross-sectional front view of the pump unit and its periphery. FIGS. 17to 19 are respectively cross sections taken along lines XVII—XVII,XVIII—XVIII, and XIX—XIX.

As illustrated in FIGS. 15 to 17, the pump unit 200 is adapted to beused in a vehicle having right and left drive wheels 283 a and 283 b towhich first and second hydraulic motors 282 a and 282 b are respectivelyconnected, and includes a first hydraulic pump 210 a and a secondhydraulic pump 210 b respectively connected to the first and secondhydraulic motors 282 a and 282 b via a first pair of hydraulic lines 284a and a second pair of hydraulic lines 284 b, and a common housing 220for accommodating these hydraulic pumps 210 a and 210 b.

The connection form between the right and left drive wheels 283 a and283 b, and the first and second hydraulic motors 282 a and 282 b meantin this embodiment includes the direct connection of the drive wheelsrespectively to those hydraulic motors, and also an operative connectionof the drive wheels respectively to the hydraulic motors via a suitablepower transmission mechanism. In FIG. 15, the reference codes 280, 281and 285 respectively represent a power source, a cooling fan and ahydraulic fluid tank.

As illustrated in FIGS. 16, 17 and 19, the first hydraulic pump 210 aand the second hydraulic pump 210 b are axial piston pumps of a variabledisplacement type, and respectively include a first pump shaft 211 a anda second pump shaft 211 b that have vertical axes and are disposedparallel to one another in the vehicle width direction within thehousing 220, a first piston unit 212 a and a second piston unit 212 bthat are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 213 a and a second cylinder block 213 bthat reciprocably support the piston units, a first angularly adjustableswash plate 214 a and a second angularly adjustable swash plate 214 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 215 a and a second control shaft 215 b thatcontrol the tilting angles of these swash plates.

The pump unit of this embodiment is of a vertical type with the firstand second pump shafts 211 a and 211 b having the vertical axes.However, The second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit200 can be of a horizontal type with the first and second pump shafts211 a and 211 b having the horizontal axes.

As best illustrated in FIG. 16, the first and second angularlyadjustable swash plates 214 a and 214 b of this embodiment are of acradle type.

As illustrated in FIGS. 16 and 19, the first control shaft 215 a and thesecond control shaft 215 b extend away from one another in the vehiclewidth direction to respectively have oppositely positioned outer ends,and inner ends extending into the housing 220 to be respectivelyconnected to arms 216 a and 216 b and hence the first and second swashplates 214 a and 214 b. The pump unit 200 with the thus arranged firstand second control shafts 215 a and 215 b is advantageous when installedon the vehicle having push-pull control levers 198 a and 198 b asillustrated in FIG. 1, since the first and second control shafts 215 aand 215 b can have the rotating shaft centers disposed parallel to thelongitudinal axis of the control levers, thereby achieving thesimplification of a link mechanism between these control shafts and thecontrol levers.

The first control shaft 215 a and the second control shaft 215 b aremore preferably located at substantially the same position with respectto the vehicle longitudinal direction, as illustrated in FIG. 16. Thethus arranged first and second control shafts 215 a and 215 b can bealigned with the control levers in the vehicle width direction, therebyachieving a more simplified structure of the link mechanism.

The pump unit 200 further includes a common center section 230 thatsupports the first and second hydraulic pumps 210 a and 210 b, and apower transmission mechanism 240 that is accommodated within the housing220 to operatively connect the first and second hydraulic pump shafts211 a and 211 b together.

The pump unit 200 with the power transmission mechanism 240 permits thesimultaneous rotation of both pump shafts 211 a and 211 b only byconnecting the power source to either one of the first and second pumpshafts 211 a and 211 b, or to the first pump shaft 211 a in thisembodiment, thereby achieving the simplified structure for the powertransmission from the power source to the pump unit 200. In thisembodiment, the power transmission mechanism 240 is in the form of agear transmission device that includes a first gear 240 a non-rotatablysupported on the lower side of the first pump shaft 211 a, and a secondgear 240 b non-rotatably supported on the lower side of the second pumpshaft 211 b in meshed engagement with the first gear 240 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

The housing 220, as illustrated in FIGS. 16 and 17, includes a firsthousing 221 for accommodating the first and second hydraulic pumps 210 aand 210 b, and a second housing 225 for accommodating the powertransmission mechanism 240.

The first housing 221 has a box shape with a first sidewall 222 disposedin the upper or lower side of the pump shafts 211 a and 211 b along thelongitudinal direction thereof, or in this embodiment in the lower sideof the pump shafts 211 a and 211 b, which will be hereinafter referredto simply as the lower side, and a peripheral wall 223 extending from aperipheral edge of the first sidewall 222 to the opposite side of thepump shafts 211 a and 211 b along the longitudinal direction thereof(i.e., the upper side of the pump shafts 211 a and 211 b in thisembodiment, which will be referred to simply as the upper side). Thefirst sidewall 222 forms bearing holes through which the first andsecond pump shafts 211 a and 211 b respectively extend. The upper sideof the first housing 221 has an end surface forming an opening throughwhich the first and second hydraulic pumps 21 a and 210 b can be placedinto the first housing 221. The opening of the first housing 221 issealed by the center section 230 in a liquid tight manner. That is, thecenter section 230 of this embodiment constitutes a part of the firsthousing 221. The first and second control shafts 215 a and 215 b extendaway from one another in the vehicle width direction to respectivelyhave outer ends protruding from the peripheral wall 223 of the firsthousing 221.

The second housing 225 is disposed in the lower side, and has a boxshape with a lower sidewall 226 forming a bearing hole through which thelower end of the first pump shaft 211 a extends and a bearing portionfor receiving the lower end of the second pump shaft 211 b, and aperipheral wall 227 extending upwardly from a peripheral edge of thelower sidewall 226. The upper side of the second housing 225 forms anopening through which the power transmission mechanism 240 can be placedinto the second housing 225.

The second housing 225 is connected to the first housing 221 in such amanner as to have the opening sealed in a liquid tight manner by thefirst sidewall 222 of the first housing 221, and form an accommodationspace of the power transmission mechanism 240 in cooperation with thefirst sidewall 222 of the first housing 221.

In the thus arranged housing 220, the first sidewall 222 of the firsthousing 221 serves as a partition wall dividing the accommodation spaceof the housing into a hydraulic pump accommodation chamber and a powertransmission mechanism accommodation chamber. The partition wall thusdefining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 240 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 212 a, 212 b, cylinder blocks213 a, 213 b, and/or other parts. In addition to this foreign mattersprevention measure, the first and second pump shafts 211 a and 211 b,which extend through the partition wall 222, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

Portions of the housing 220, through which the respective shafts 211 a,215 a and 215 b extend, are sealed by any suitable sealing means in aliquid tight manner, thereby allowing the housing 220 to serve as thehydraulic fluid tank 285.

The first sidewall 222 serving as the partition wall preferably forms ahydraulic fluid communication hole 222 a for communication between thehydraulic pump accommodation chamber and the power transmissionmechanism accommodation chamber with a filter 222 b provided in the holefor preventing the intrusion of the foreign matters into the hydraulicpump accommodation chamber. The thus formed hydraulic fluidcommunication hole 222 a can omit the necessity of separately feedingthe lubricant to the power transmission mechanism 240, with the resultthat the power transmission mechanism 240 can be lubricated with thehydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

In this embodiment, the first and second angularly adjustable swashplates 214 a and 214 b are of a cradle type, as illustrated in FIG. 17.Therefore, when the partition wall 222 forms, on its side facing thehydraulic pumps 210 a, 210 b, spherical concave surfaces 222 crespectively adapted to spherical convex surfaces 216 formed in the rearsides of the swash plates 214 a and 214 b, which rear sides beingopposite to the surfaces facing the piston units 212 a and 212 b, thespherical concave surfaces 222 c can slidingly guide the sphericalconvex surfaces 216 of the swash plates 214 a and 214 b. The swashplates thus can securely rest on the spherical concave surfaces 222 c.Although FIG. 17 illustrates only the portion of the partition wall 222corresponding to the first angularly adjustable swash plate 214 a, it isa matter of course that the portion of the partition wall 222corresponding to the second angularly adjustable swash plate 214 b formsthe spherical concave surface 222 c.

In this embodiment, the first sidewall 222 of the first housing 221serves as the partition wall. Alternatively, a partitioning means maytake various forms, as long as it can produce the same effect asdescribed above. For example, a separately prepared partition wall maybe mounted in a housing having a simple cylindrical box shape (see FIG.14).

Now, the description will be made for the center section 230. Asillustrated in FIG. 18, the center section 230 forms a first pair ofhydraulic passages 231 a for the first hydraulic pump communicating withthe first piston unit. The first pair of hydraulic passages 231 arespectively have first ends opening to the outside of the centersection 230 to form a first pair of inlet/outlet ports 232 a serving asconnection ports for connection with the first pair of hydraulic lines284 a extending between the first hydraulic motor and the center section230 (see FIG. 15).

Similarly, the center section 230 forms a second pair of hydraulicpassages 231 b for the second hydraulic pump communicating with thesecond piston unit. The second pair of hydraulic passages 231 brespectively have first ends forming a second pair of inlet/outlet ports232 b serving as connection ports for connection with the second pair ofhydraulic lines 284 b (see FIG. 15).

As described above, the common center section 230 thus forms all thefirst and second pairs of inlet/outlet ports 232 a and 232 b serving asthe connection ports for connection with the first and second pairs ofhydraulic lines 284 a and 284 b. Whereby, the piping work between thehydraulic pumps 210 a and 210 b, and the hydraulic motors 282 a and 282b can be facilitated. The first and second pairs of inlet/outlet portsare more preferably formed in the same side of the center section 230,as illustrated in FIG. 18, thereby further facilitating the piping work.

The center section 230 also forms a common charging passage 233 forfeeding a pressurized hydraulic fluid to the first pair of hydrauliclines 284 a and the second pair of hydraulic lines 284 b. The chargingpassage 233 has a first end opening to the outside of the center section230 to form an inlet port for charging 234. In this embodiment, thefirst pump shaft 211 a, as illustrated in FIG. 16, has an extensionextending further from the upper end thereof to be located above thecenter section 230, thereby supporting a charge pump 250 via theextension, and connecting an outlet port 251 of the charge pump 250 tothe inlet port 234. The outlet port 251 of the charge pump alsocommunicates with a pressure relief line 253 having a charge reliefvalve 252 therein. The charge relief valve 252 is designed to adjust thehydraulic pressure in the charging passage 233 (see FIG. 15). Thepressure relief line 253 has a rear end communicating via a drain port235 formed in the center section 230 with the housing 220 serving alsoas the hydraulic fluid tank 285. The reference codes 255 and 256 inFIGS. 16 and 17 respectively represent an inlet port of the charge pump,and an inlet port communicating with the inlet port of the charge pumpand connected to the hydraulic fluid tank 285 through a suitableconduit.

On the other hand, the charging passage 233, as illustrated in FIG. 18,has a second end communicating with the first pair of hydraulic passages231 a and the second pair of hydraulic passages 231 b via check valves261 a, 261 b, 261 c and 261 d so as to allow the pressurized hydraulicfluid to be fed from the common charging passage 233 into a lowerpressure line of the first pair of hydraulic lines 284 a and a lowerpressure line of the second pairs of hydraulic lines 284 b, whilepreventing the pressurized hydraulic fluid from flowing in the reversedirection.

Bypass lines 262 a and 262 b having throttle valves are formed betweenat least one of the first pair of hydraulic passages 231 a and thecharging passage 233, and between at least one of the second pair ofhydraulic passages 231 b and the charging passage 233 (see FIGS. 15 and18).

The bypass lines 262 a and 262 b are designed to assure theneutralization of the hydraulic pumps 210 a and 210 b. Specifically,even if the swash plates 214 a and 214 b of the hydraulic pumps 210 aand 210 b tilt from the neutral positions by a small angle, there occursthe pressure difference between the first pair of hydraulic lines 284 a,and/or between the second pair of hydraulic lines 284 b. This pressuredifference causes the rotation of the hydraulic motors 282 a and 282 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 214 a and 214 b due to assembling errors or the like causes anunintentional rotation of the hydraulic motors 282 a and 282 b. On thecontrary, the bypass lines 262 a and 262 b, as described above, allowthe pressurized hydraulic fluid to leak therethrough from the first pairof hydraulic lines 284 a and the second pair of hydraulic lines 284 b.Thus, the pressure difference between the pair of first hydraulic lines284 a and/or between the second pair of hydraulic lines 284 b caneffectively be limited, thereby effectively avoiding the unintentionalrotation of the hydraulic motors 282 a and 282 b, even for the swashplates 214 a and 214 b having the actual neutral position displaced fromthe design neutral position due to the assembling errors or the like.

In view of transmission efficiency between the hydraulic pumps 210 a,210 b and the hydraulic motors 282 a, 282 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 284 a, 284 b through the bypass lines 262 a, 262 b is notpreferable. Therefore, the bypass lines 262 a, 262 a are preferablyprovided in portions from the charging passage 233 to one of the firstpair of hydraulic passages 231 a, and to one of the second pair ofhydraulic passages 231 b.

The check valves 261 a, 261 b, 261 c and 261 d are more preferablyprovided with release means 262 to forcibly bring the first pair ofhydraulic passages 231 a into communication with one another, and thesecond pair of hydraulic passages 231 b into communication with oneanother, if an emergency arises, as illustrated in FIG. 18. The releasemeans 262 are designed to easily move the vehicle, when the vehicle mustforcibly be moved or the vehicle wheels must forcibly be rotated by manpower or the like due to the disorder of the power source 280, thehydraulic pumps 210 a, 210 b or the like. Specifically, when the vehiclewheels connected to the hydraulic motors 282 a and 282 b are forciblyrotated with the first pair of hydraulic lines 284 a and/or the secondpair of hydraulic lines 284 b lying in the closing state, there occursthe pressure difference between the first pair of hydraulic lines 284 a,and between the second pair of hydraulic lines 284 b. As a result, thevehicle is hardly moved, or the vehicle wheels are hardly rotated. Onthe contrary, the release means can easily achieve the communicationsbetween the first pair of hydraulic passages 231 a, and between thesecond pair of hydraulic passages 231 b by mechanically releasing allthe check valves 261 a to 261 d. Whereby, the vehicle can easily bemoved by man power or the like.

As illustrated in FIG. 18, all the release means 263 are preferablydisposed in the same side of the center section 230, so that the linkmechanism linking these release means 263 for operation of the same canhave a simplified structure.

The pump unit of this embodiment includes the charge pump 250 toforcibly feed the pressurized hydraulic fluid into the inlet port forcharging 234. As an alternative to the arrangement using the chargepump, the pump unit may have an arrangement where the inlet port 234 isconnected to the hydraulic fluid tank, thereby allowing the hydraulicfluid to spontaneously flow into the inlet port 234 when the pressure ina lower pressure line of the first pair of hydraulic lines 284 a and/orthe pressure in a lower pressure line of the second pair of hydrauliclines 284 b drops from a predetermined value.

Fourth Embodiment

Another embodiment of the pump unit according to the second aspect ofthe present invention will be hereinafter described with reference toFIGS. 20 to 23. In the following description, corresponding or identicalparts to those of the third embodiment have been given the samereference characters or those with primes (′) to omit a detaileddescription thereof.

FIGS. 20 and 21 are respectively a longitudinal cross-sectional sideview, and a longitudinal cross-sectional front view of the pump unitaccording to this embodiment. FIGS. 22 and 23 are respectively crosssections taken along lines XXII—XXII and XXIII—XXIII in FIG. 20.

As illustrated in FIGS. 20 and 21, the pump unit 200′ of this embodimentincludes the first hydraulic pump 210 a′ and the second hydraulic pump210 b′, both of which are disposed parallel to one another along thevehicle longitudinal direction, and the first and second hydraulic pumps210 a′ and 210 b′ respectively having the angularly adjustable swashplates 214 a′ and 214 b′ of trunnion type.

As illustrated in FIGS. 21 and 23, the first control shaft 215 a and thesecond control shaft 215 b extend away from one another along thevehicle width direction, in the same manner as those of theaforementioned embodiments.

As illustrated in FIG. 22, both first and second pairs of inlet/outletports 232 a and 232 b are formed in the same side of the center section230′. The pump unit of this embodiment is also arranged so that thepressurized hydraulic fluid can be fed into the first pair of hydraulicpassages 231 a and the second pair of hydraulic passages 231 b via theinlet port for charging 234 and the charging passage 233 communicatingwith the port 234.

The pump unit 200′ having the first and second hydraulic pumps 210 a′and 210 b′ arranged parallel to one another along the longitudinaldirection has the first control shaft 215 a displaced from the secondcontrol shaft 215 b with respect to the vehicle longitudinal direction,as illustrated in FIG. 23. This displacement can be easily compensatedby using arms or other suitable linking means.

The pump unit 200′ having the above arrangement also produces the sameeffects as those of the third embodiment.

Fifth Embodiment

One embodiment of the pump unit according to the third aspect of thepresent invention will be hereinafter described with reference to FIGS.24 to 28. FIG. 24 is a hydraulic circuit diagram of the vehicle to whicha pump unit 300 of this embodiment is applied. FIG. 25 is a longitudinalcross-sectional front view of the pump unit and its periphery. FIGS. 26to 28 are respectively cross sections taken along lines XXVI—XXVI,XXVII—XXVII and XXVIII—XXVIII in FIG. 25.

As illustrated in FIGS. 24 to 26, the pump unit 300 is adapted to beused in a vehicle having right and left drive wheels 383 a and 383 b towhich first and second hydraulic motors 382 a and 382 b are respectivelyconnected, and includes a first hydraulic pump 310 a and a secondhydraulic pump 310 b respectively connected to the first and secondhydraulic motors 382 a and 382 b via a first pair of hydraulic lines 384a and a second pair of hydraulic lines 384 b, and a common housing 320for accommodating these hydraulic pumps 310 a and 310 b.

The connection form between the right and left drive wheels 383 a and383 b, and the first and second hydraulic motors 382 a and 382 b meantin this embodiment includes the direct connection of the drive wheelsrespectively to those hydraulic motors, and also an operative connectionof the drive wheels respectively to the hydraulic motors via a suitablepower transmission mechanism. In FIG. 24, the reference codes 380, 381and 385 respectively represent a power source, a cooling fan and ahydraulic fluid tank.

As illustrated in FIGS. 25, 26 and 28, the first hydraulic pump 310 aand the second hydraulic pump 310 b are axial piston pumps of a variabledisplacement type, and respectively include a first pump shaft 311 a anda second pump shaft 311 b that have vertical axes and are disposedparallel to one another in the vehicle width direction within thehousing 320, a first piston unit 312 a and a second piston unit 312 bthat are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 313 a and a second cylinder block 313 bthat reciprocably support the piston units, a first angularly adjustableswash plate 314 a and a second angularly adjustable swash plate 314 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 315 a and a second control shaft 315 b thatcontrol the tilting angles of these swash plates.

The pump unit of this embodiment is of a vertical type with the firstand second pump shafts 311 a and 311 b having the vertical axes.However, the second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit300 can be of a horizontal type with the first and second pump shafts311 a and 311 b having the horizontal axes.

As best illustrated in FIG. 25, the first and second angularlyadjustable swash plates 314 a and 314 b of this embodiment are of acradle type.

As illustrated in FIGS. 25 and 28, the first control shaft 315 a and thesecond control shaft 315 b extend away from one another in the vehiclewidth direction to respectively have oppositely positioned outer ends,and inner ends extending into the housing 320 to be respectivelyconnected to arms 316 a and 316 b and hence the first and second swashplates 314 a and 314 b. The pump unit 300 with the thus arranged firstand second control shafts 315 a and 315 b is advantageous when installedon the vehicle having push-pull control levers 198 a and 198 b asillustrated in FIG. 1, since the first and second control shafts 315 aand 315 b can have the rotating shaft centers disposed parallel to thelongitudinal axis of the control levers, thereby achieving thesimplification of a link mechanism between these control shafts and thecontrol levers.

The first control shaft 315 a and the second control shaft 315 b aremore preferably located at the same position with respect to the vehiclelongitudinal direction, as illustrated in FIG. 25. The thus arrangedfirst and second control shafts 315 a and 315 b can be aligned with thecontrol levers in the vehicle width direction, thereby achieving a moresimplified structure of the link mechanism.

The pump unit 300 further includes a common center section 330 t hatsupports the first and second hydraulic pumps 310 a and 310 b, and apower transmission mechanism 340 that is accommodated within the housing320 to operatively connect the first and second hydraulic pump shafts311 a and 311 b together.

The pump unit 300 with the power transmission mechanism 340 permits thesimultaneous rotation of both pump shafts 311 a and 311 b only byconnecting the power source to either one of the first and second pumpshafts 311 a and 311 b, or to the first pump shaft 311 a in thisembodiment, thereby achieving the simplified structure for the powertransmission from the power source to the pump unit 300. In thisembodiment, the power transmission mechanism 340 is in the form of agear transmission device that includes a first gear 340 a non-rotatablysupported on the lower side of the first pump shaft 311 a, and a secondgear 340 b non-rotatably supported on the lower side of the second pumpshaft 311 b in meshed engagement with the first gear 340 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

The housing 320, as illustrated in FIGS. 25 and 26, includes a firsthousing 321 for accommodating the first and second hydraulic pumps 310 aand 310 b, and a second housing 325 for accommodating the powertransmission mechanism 340.

The first housing 321 has a box shape with a first side wall 322disposed in the upper or lower side of the pump shafts 311 a and 311 balong the longitudinal direction thereof, or in this embodiment in thelower side of the pump shafts 311 a and 311 b, which will be hereinafterreferred to simply as the lower side, and a peripheral wall 323extending from a peripheral edge of the first sidewall 322 to theopposite side of the pump shafts 311 a and 311 b along the longitudinaldirection thereof (i.e., the upper side of the pump shafts 311 a and 311b in this embodiment, which will be referred to simply as the upperside). The first sidewall 322 forms bearing holes through which thefirst and second pump shafts 311 a and 311 b respectively extend. Theupper side of the first housing 321 has an end surface forming anopening through which the first and second hydraulic pumps 310 a and 310b can be placed into the first housing 321. The opening of the firsthousing 321 is sealed by the center section 330 in a liquid tightmanner. That is, the center section 330 of this embodiment constitutes apart of the first housing 321. The first and second control shafts 315 aand 315 b extend away from one another in the vehicle width direction torespectively have outer ends protruding from the peripheral wall 323 ofthe first housing 321.

The second housing 325 is disposed in the lower side, and has a boxshape with a lower sidewall 326 forming a bearing hole through which thelower end of the first pump shaft 311 a extends and a bearing portionfor receiving the lower end of the second pump shaft 311 b, and aperipheral wall 327 extending upwardly from a peripheral edge of thelower sidewall 326. The upper side of the second housing 325 forms anopening through which the power transmission mechanism 310 can be placedinto the second housing 325.

The second housing 325 is connected to the first housing 321 in such amanner as to have the opening sealed in a liquid tight manner by thefirst sidewall 322 of the first housing 321, and form an accommodationspace of the power transmission mechanism 340 in cooperation with thefirst sidewall 322 of the first housing 321.

In the thus arranged housing 320, the first sidewall 322 of the firsthousing 321 serves as a partition wall dividing the accommodation spaceof the housing into a hydraulic pump accommodation chamber and a powertransmission mechanism accommodation chamber. The partition wall thusdefining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 340 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 312 a, 312 b, cylinder blocks313 a, 313 b, and/or other parts. In addition to this foreign mattersprevention measure, the first and second pump shafts 311 a and 311 b,which extend through the partition wall 322, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

Portions of the housing 320, through which the respective shafts 311 a,315 a and 315 b extend, are sealed by any suitable sealing means in aliquid tight manner, thereby allowing the housing 320 to serve as thehydraulic fluid tank 385.

The first sidewall 322 serving as the partition wall preferably forms ahydraulic fluid communication hole 322 a for communication between thehydraulic pump accommodation chamber and the power transmissionmechanism accommodation chamber with a filter 322 b provided in the holefor preventing the intrusion of the foreign matters into the hydraulicpump accommodation chamber. The thus formed hydraulic fluidcommunication hole 322 a can omit the necessity of separately feedingthe lubricant to the power transmission mechanism 340, with the resultthat the power transmission mechanism 340 can be lubricated with thehydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

In this embodiment, the first and second angularly adjustable swashplates 314 a and 314 b are of a cradle type, as illustrated in FIG. 26.Therefore, when the partition wall 322 forms, on its side facing thehydraulic pumps 310 a, 310 b, spherical concave surfaces 322 crespectively adapted to spherical convex surfaces 316 formed in the rearsides of the swash plates 314 a and 314 b, which rear sides beingopposite to the surfaces facing the piston units 312 a and 312 b, thespherical concave surfaces 322 c can slidingly guide the sphericalconvex surfaces 316 of the swash plates 314 a and 314 b. The swashplates thus can securely rest on the spherical concave surfaces 322 c.Although FIG. 26 illustrates only the portion of the partition wall 322corresponding to the first angularly adjustable swash plate 314 a, it isa matter of course that the portion of the partition wall 322corresponding to the second angularly adjustable swash plate 314 b formsthe spherical concave surface 322 c.

In this embodiment, the first sidewall 322 of the first housing 321serves as the partition wall. Alternatively, a partitioning means maytake various forms, as long as it can produce the same effect. Forexample, a separately prepared partition wall may be mounted in ahousing having a simple cylindrical box shape (see FIG. 14).

Now, the description will be made for the center section 330. Asillustrated in FIG. 27, the center section 330 forms a first pair ofhydraulic passages 331 a for the first hydraulic pump communicating withthe first piston unit. The first pair of hydraulic passages 331 arespectively have first ends opening to the outside of the centersection 330 to form a first pair of inlet/outlet ports 332 a serving asconnection ports for connection with the first pair of hydraulic lines384 a extending between the first hydraulic motor and the center section330 (see FIG. 24).

Similarly, the center section 330 forms a second pair of hydraulicpassages 331 b for the second hydraulic pump communicating with thesecond piston unit. The second pair of hydraulic passages 331 brespectively have first ends forming a second pair of inlet/outlet ports332 b serving as connection ports for connection with the second pair ofhydraulic lines 384 b (see FIG. 24).

As described above, the common center section 330 thus forms all thefirst and second pairs of inlet/outlet ports 332 a and 332 b serving asthe connection ports for connection with the first and second pairs ofhydraulic lines 384 a and 384 b. Whereby, the conduit work between thehydraulic pumps 310 a and 310 b, and the hydraulic motors 382 a and 382b can be facilitated. The first and second pairs of inlet/outlet portsare more preferably formed in the same side of the center section 330,as illustrated in FIG. 27, thereby further facilitating the conduitwork.

The center section 330 also forms a common charging passage 333 forfeeding a pressurized hydraulic fluid to the first pair of hydrauliclines 384 a and the second pair of hydraulic lines 384 b. The chargingpassage 333 has a first end opening to the outside of the center section330 to form an inlet port for charging 334. In this embodiment, thefirst pump shaft 311 a, as illustrated in FIG.25, has an extensionextending further from the upper end thereof to be located above thecenter section 330, thereby supporting a charge pump 350 via theextension, and connecting an outlet port 351 of the charge pump 350 tothe inlet port 334. The outlet port 351 of the charge pump alsocommunicates with a pressure relief line 353 having a charge reliefvalve 352 therein. The charge relief valve 352 is designed to adjust thehydraulic pressure in the charging passage 333 (see FIG. 24). Thepressure relief line 353 has a rear end communicating via a drain port335 formed in the center section 330 with the housing 320 serving alsoas the hydraulic fluid tank 385. The reference codes 355 and 356 inFIGS. 25 and 26 respectively represent an inlet port of the charge pump,and an inlet port communicating with the inlet port of the charge pumpand connected to the hydraulic fluid tank 385 through a suitableconduit.

On the other hand, the charging passage 333, as illustrated in FIG. 27,has a second end communicating with the first pair of hydraulic passages331 a and the second pair of hydraulic passages 331 b via check valves361 a, 361 b, 361 c and 361 d so as to allow the pressurized hydraulicfluid to be fed from the common charging passage 333 into a lowerpressure line of the first pair of hydraulic lines 384 a and a lowerpressure line of the second pair of hydraulic lines 384 b, whilepreventing the pressurized hydraulic fluid from flowing in the reversedirection.

Bypass lines 362 a and 362 b having throttle valves are formed betweenat least one of the first pair of hydraulic passages 331 a and thecharging passage 333, and between at least one of the second pair ofhydraulic passages 331 b and the charging passage 333 (see FIGS. 24 and27).

The bypass lines 362 a and 362 b are designed to assure theneutralization of the hydraulic pumps 310 a and 310 b. Specifically,even if the swash plates 314 a and 314 b of the hydraulic pumps 310 aand 310 b tilt from the neutral positions by a small angle, there occursthe pressure difference between the first pair of hydraulic lines 384 a,and/or between the second pair of hydraulic lines 384 b. This pressuredifference causes the rotation of the hydraulic motors 383 a and 383 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 314 a and 314 b due to assembling errors or the like causes anunintentional rotation of the hydraulic motors 383 a and 383 b. On thecontrary, the bypass lines 362 a and 362 b, as described above, allowthe pressurized hydraulic fluid to leak therethrough from the first pairof hydraulic lines 384 a and the second pair of hydraulic lines 384 b.Thus, the pressure difference between the pair of first hydraulic lines384 a and/or between the second pair of hydraulic lines 384 b caneffectively be limited, thereby effectively avoiding the unintentionalrotation of the hydraulic motors 382 a and 382 b, even for the swashplates 314 a and 314 b having the actual neutral position displaced fromthe design neutral position due to the assembling errors or the like.

In view of transmission efficiency between the hydraulic pumps 310 a,310 b and the hydraulic motors 382 a, 382 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 384 a, 384 b through the bypass lines 362 a, 362 b is notpreferable. Therefore, the bypass lines 362 a, 362 b are preferablyprovided in portions from the charging passage 333 to one of the firstpair of hydraulic passages 33 1 a, and to one of the second pair ofhydraulic passages 331 b.

The check valves 361 a, 361 b, 361 c and 361 d are more preferablyprovided with release means 362 to forcibly bring the first pair ofhydraulic passages 331 into communication with one another, and thesecond pair of hydraulic passages 331 b into communication with oneanother, if an emergency arises, as illustrated in FIG. 27. The releasemeans 362 are designed to easily move the vehicle, w hen the vehiclemust forcibly be moved or the vehicle wheels must forcibly be rotated byman power or the like due to the disorder of the power source 380, thehydraulic pumps 310 a, 310 b or the like. Specifically, when the vehiclewheels connected to the hydraulic motors 382 a and 382 b are forciblyrotated with the first pair of hydraulic lines 384 a and/or the secondpair of hydraulic lines 384 b lying in the closing state, there occursthe pressure difference between the first pair of hydraulic lines 331 a,and between the second pair of hydraulic lines 331 b. As a result, thevehicle is hardly moved, or the vehicle wheels are hardly rotated. Onthe contrary, the release means can easily achieve the communicationsbetween the first pair of hydraulic lines 384 a, and between the secondpair of hydraulic lines 384 b by mechanically releasing all the checkvalves 361 a to 361 d. Whereby, the vehicle can easily be moved by manpower or the like.

As illustrated in FIG. 27, all the release means 363 are preferablydisposed in the same side of the center section 330, so that the linkmechanism for linking these release means 363 can have a simplifiedstructure.

The pump unit of this embodiment includes the charge pump 350 toforcibly feed the pressurized hydraulic fluid into the inlet port forcharging 334. As an alternative to the arrangement using the chargepump, the pump unit may have an arrangement where the inlet port 334 isconnected to the hydraulic fluid tank, thereby allowing the hydraulicfluid to spontaneously flow into the inlet port 334 when the pressure ina lower pressure line of the first pair of hydraulic lines 384 a and/orthe pressure in a lower pressure line of the second pair of hydrauliclines 384 b drops from a predetermined value.

Sixth Embodiment

Another embodiment of the pump unit according to the third aspect. ofthe present invention will be hereinafter described with reference toFIGS. 29 to 32. In the following description, corresponding or identicalparts to those of the fifth embodiment have been given the samereference characters or those with primes (′) to omit a detaileddescription thereof.

FIGS. 29 and 30 are respectively a longitudinal cross-sectional sideview, and a longitudinal cross-sectional front view of the pump unitaccording to this embodiment. FIGS. 31 and 32 are respectively crosssections taken along lines XXXI—XXXI, and XXXII—XXXII in FIG. 29.

As illustrated in FIGS. 29 and 30, the pump unit 300′ of this embodimentincludes the first hydraulic pump 310 a′ and the second hydraulic pump310 b′, both of which are disposed parallel to one another along thevehicle longitudinal direction, and the first and second hydraulic pumps310 a′ and 310 b′ respectively having the angularly adjustable swashplates 314 a′ and 314 b′ of trunnion type.

As illustrated in FIGS. 30 and 32, the first control shaft 315 a and thesecond control shaft 315 b extend away from one another along thevehicle width direction, in the same manner as those of theaforementioned embodiments.

As illustrated in FIG. 31, both first and second pairs of inlet/outletports 332 a and 332 b are formed in the same side of the center section330′. The pump unit of this embodiment is also arranged so that thepressurized hydraulic fluid can be fed into the first pair of hydraulicpassages 331 a and the second pair of hydraulic passages 331 b via theinlet port for charging 334 and the charging passage 333 communicatingwith the port 334.

The pump unit 300′ having the first and second hydraulic pumps 310 a′and 310 b′ arranged parallel to one another along the longitudinaldirection has the first control shaft 315 a displaced from the secondcontrol shaft 315 b with respect to the vehicle longitudinal direction,as illustrated in FIG. 32. This displacement can be easily compensatedby using arms or other suitable linking means.

The pump unit 300′ having the above arrangement also produces the sameeffects as those of the fifth embodiment.

Seventh Embodiment

One embodiment of the pump unit according to the fourth aspect of thepresent invention will be hereinafter described with reference to theaccompanying drawings. FIG. 33 is a hydraulic circuit diagram of thevehicle to which a pump unit 400 of this embodiment is applied. FIG. 34is a longitudinal cross-sectional side view of the pump unit and itsperiphery. FIGS. 35 and 36 are respectively cross sections taken alonglines XXXV—XXXV, and XXXVI—XXXVI.

As illustrated in FIGS. 33 to 35, the pump unit 400 is adapted to beused in a vehicle having right and left drive wheels 483 a and 483 b towhich first and second hydraulic motors 482 a and 482 b are respectivelyconnected, and includes a first hydraulic pump 410 a and a secondhydraulic pump 410 b respectively connected to the first and secondhydraulic motors 482 a and 482 b via a first pair of hydraulic lines 484a and a second pair of hydraulic lines 484 b, and a common housing 420for accommodating these hydraulic pumps 410 a and 410 b.

The connection form between the right and left drive wheels 483 a and483 b, and the first and second hydraulic motors 482 a and 482 b meantin this embodiment includes the direct connection of the drive wheelsrespectively to those hydraulic motors, and also an operative connectionof the drive wheels respectively to the hydraulic motors via a suitablepower transmission mechanism. In FIG. 33, the reference codes 480, 481and 486 respectively represent a power source, a cooling fan and afilter.

As illustrated in FIGS. 34 to 36, the first hydraulic pump 410 a and thesecond hydraulic pump 410 b are axial piston pumps of a variabledisplacement type, and respectively include a first pump shaft 411 a anda second pump shaft 411 b that have vertical axes and are disposedparallel to one another in the vehicle width direction within thehousing 420, a first piston unit 412 a and a second piston unit 412 bthat are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 413 a and a second cylinder block 413 bthat reciprocably support the piston units, a first angularly adjustableswash plate 414 a and a second angularly adjustable swash plate 414 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 415 a and a second control shaft 415 b thatcontrol the tilting angles of these swash plates.

The pump unit of this embodiment is of a vertical type with the firstand second pump shafts 411 a and 411 b having the vertical axes.However, the second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit400 can be of a horizontal type with the first and second pump shafts411 a and 411 b having the horizontal axes.

As best illustrated in FIG. 34, the first and second angularlyadjustable swash plates 414 a and 414 b of this embodiment are of acradle type.

As illustrated in FIGS. 35 and 36, the first control shaft 415 a and thesecond control shaft 415 b extend away from one another in the vehiclewidth direction to respectively have oppositely positioned outer ends,and inner ends extending into the housing 420 to be respectivelyconnected to arms 416 a and 416 b and hence the first and second swashplates 414 a and 414 b. The pump unit 400 with the thus arranged firstand second control shafts 415 a and 415 b is advantageous when installedon the vehicle having push-pull control levers 198 a and 198 b asillustrated in FIG. 1, since the first and second control shafts 415 aand 415 b can have the rotating shaft centers disposed parallel to thelongitudinal axis of the control levers, thereby achieving thesimplification of a link mechanism between these control shafts and thecontrol levers.

The first control shaft 415 a and the second control shaft 415 b aremore preferably located at substantially the same position with respectto the vehicle longitudinal direction, as illustrated in FIG. 34. Thethus arranged first and second control shafts 415 a and 415 b can bealigned with the control levers in the vehicle width direction, therebyachieving a more simplified structure of the link mechanism.

The pump unit 400 further includes a common center section 430 thatsupports the first and second hydraulic pumps 410 a and 410 b, and apower transmission mechanism 440 that is accommodated within the housing420 to operatively connect the first and second hydraulic pump shafts411 a and 411 b together.

The pump unit 400 with the power transmission mechanism 440 permits thesimultaneous rotation of both pump shafts 411 a and 411 b only byconnecting the power source to either one of the first and second pumpshafts 411 a and 411 b, or to the first pump shaft 411 a in thisembodiment, thereby achieving the simplified structure for the powertransmission from the power source to the pump unit 400. In thisembodiment, the power transmission mechanism 440 is in the form of agear transmission device that includes a first gear 440 a non-rotatablysupported on the lower side of the first pump shaft 411 a, and a secondgear 440 b non-rotatably supported on the lower side of the second pumpshaft 411 b in meshed engagement with the first gear 440 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

The housing 420, as illustrated in FIGS. 34 and 35, includes a firsthousing 421 for accommodating the first and second hydraulic pumps 410 aand 410 b, and a second housing 422 for accommodating the powertransmission mechanism 140.

The first housing 421 has a box shape with a first sidewall 421 adisposed in the upper or lower side of the pump shafts 411 a and 411 balong the longitudinal direction thereof, or in this embodiment in thelower side of the pump shafts 411 a and 411 b, which will be hereinafterreferred to simply as the lower side, and a peripheral wall 421 bextending from a peripheral edge of the first sidewall 421 a to theopposite side of the pump shafts 411 a and 411 b along the longitudinaldirection thereof (i.e., the upper side of the pump shafts 411 a and 411b in this embodiment, which will be referred to simply as the upperside). The first sidewall 421 a forms bearing holes through which thefirst and second pump shafts 411 a and 411 b) respectively extend. Theupper side of the first housing 421 has an end surface forming anopening through which the first and second hydraulic pumps 410 a and 410b can be placed into the first housing 421. The opening of the firsthousing 421 is sealed by the center section 430 in a liquid tightmanner. That is, the center section 430 of this embodiment constitutes apart of the first housing 421. The first and second control shafts 415 aand 415 b extend away from one another in the vehicle width direction torespectively have outer ends protruding from the peripheral wall 421 bof the first housing 421.

The second housing 422 is disposed in the lower side, and has a boxshape with a lower sidewall 422 a forming a bearing hole through whichthe lower end of the first pump shaft 411 a extends and a bearingportion for receiving the lower end of the second pump shaft 411 b, anda peripheral wall 422 b extending upwardly from a peripheral edge of thelower sidewall 422 a. The upper side of the second housing 422 forms anopening through which the power transmission mechanism 440 can be placedinto the second housing 422.

The second housing 422 is connected to the first housing 421 in such amanner as to have the opening sealed in a liquid tight manner by thefirst sidewall 421 a of the first housing 421, and form an accommodationspace of the power transmission mechanism 440 in cooperation with thefirst sidewall 421 a of the first housing 421.

In the thus arranged housing 420, the first sidewall 421 a of the firsthousing 421 serves as a partition wall dividing the accommodation spaceof the housing into a hydraulic pump accommodation chamber and a powertransmission mechanism accommodation chamber. The partition wall thusdefining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 440 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 412 a, 412 b, cylinder blocks413 a, 413 b, and/or other parts. In addition to this foreign mattersprevention measure, the first and second pump shafts 411 a and 411 b,which extend through the partition wall 421 a, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

Portions of the housing 420, through which the respective shafts 411 a,415 a and 415 b extend, are sealed by any suitable sealing means in aliquid tight manner, thereby allowing the housing 420 to serve as thehydraulic fluid tank.

The first sidewall 421 a serving as the partition wall preferably formsa hydraulic fluid communication hole 423 a for communication between thehydraulic pump accommodation chamber and the power transmissionmechanism accommodation chamber with a filter 423 b provided in the holefor preventing the intrusion of the foreign matters into the hydraulicpump accommodation chamber. The thus formed hydraulic fluidcommunication hole 423 a can omit the necessity of separately feedingthe lubricant to the power transmission mechanism 440, with the resultthat the power transmission mechanism 440 can be lubricated with thehydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

In this embodiment, the first and second angularly adjustable swashplates 414 a and 414 b are of a cradle type, as illustrated in FIG. 34.Therefore, when the partition wall 421 a forms, on its side facing thehydraulic pumps 410 a, 410 b, spherical concave surfaces 424respectively adapted to spherical convex surfaces 419 formed in the rearsides of the swash plates 414 a and 414 b, which rear sides beingopposite to the surfaces facing the piston units 412 a and 412 b, thespherical concave surfaces 424 can slidingly guide the spherical convexsurfaces 419 of the swash plates 414 a and 414 b. The swash plates thuscan securely rest on the spherical concave surfaces 424. Although FIG.34 illustrates only the portion of the partition wall 421 acorresponding to the first angularly adjustable swash plate 414 a, it isa matter of course that the portion of the partition wall 421 acorresponding to the second angularly adjustable swash plate 414 b formsthe spherical concave surface 424.

In this embodiment, the first sidewall 421 a of the first housing 421serves as the partition wall Alternatively, a partitioning means maytake various forms, as long as it can produce the same effect asdescribed above. For example, a separately prepared partition wall maybe mounted in a housing having a simple cylindrical box shape (see FIG.14).

Now, the description will be made for the center section 430. FIG. 37 isan enlarged view of a portion XXXVII in FIG. 34. FIGS. 38 and 39 arerespectively cross sections taken along lines XXXVIII—XXXVIII, andXXXIX—XXXIX in FIG. 37. FIG. 40 is a cross section taken along linesXXXX—XXXX in FIG. 39.

As best illustrated in FIG. 39, the center section 430 forms a firstpair of hydraulic passages 431 a respectively having first endscommunicating with the first piston unit and second ends opening to theoutside of the center section 430 to form a first pair of inlet/outletports 432 a serving as connection ports for connection with the firstpair of hydraulic lines 484 a (see FIG. 33).

The center section 430 also forms a second pair of hydraulic passages431 b having first ends communicating with the second piston unit andsecond ends opening to the outside of the center section 430 to form asecond pair of inlet/outlet ports 432 b serving as connection ports forconnection with the second pair of hydraulic lines 484 b (see FIG. 33).

As described above, the common center section 430 thus forms all thefirst and second pairs of inlet/outlet ports 432 a and 432 b serving asthe connection ports for connection with the first and second pairs ofhydraulic lines 484 a and 484 b. Whereby, the piping work between thehydraulic pumps 410 a and 410 b, and the hydraulic motors 482 a and 482b can be facilitated. The first and second pairs of inlet/outlet ports432 a and 432 b are more preferably formed in the same side of thecenter section 430, as illustrated in FIGS. 39 and 40, thereby furtherfacilitating the piping work.

The center section 430, as illustrated in FIGS. 37 to 40, also forms acommon charging passage 433 having a first end opening to the outside ofthe center section 430 to form an inlet port for charging 434 serving asan inlet for the hydraulic fluid to be replenished, and a second endcommunicating with the first pair of hydraulic passages 431 a and thesecond pair of hydraulic passages 431 b via check valves 461 a, 461 b,461 c and 461 d.

In this embodiment, the first pump shaft 411 a, as best illustrated inFIGS. 34 and 37, has an extension extending upwards from the upper endthereof to be located above the center section 430, thereby supporting acharge pump 450 via the extension. The charge pump 450 has an uppersurface with a cartridge filter 486 detachably mounted thereto, throughwhich the pressurized hydraulic fluid is fed from an outlet port 451 ofthe charge pump 450 to the inlet port for charging 434.

The cartridge filter 486 can be provided at the suction side of thecharge pump 450.

The charging passage 433 is connected via a relief valve 452 to apressure relief line 453 communicating with the housing. The reliefvalve 452 regulates the hydraulic pressure of the charging passage 433(see FIGS. 33 and 40).

In this embodiment, the pressure relief line 453 is formed in a chargepump casing 459 mounted on the upper surface of the center section 430to be communicated with the first housing 421 via a drain port 435formed in the center section 430. However, the present invention is notnecessarily limited to this arrangement. That is, the pressure reliefline 453 can be formed in the center section 430.

Reference codes 455, and 456 in FIGS. 34, 35 and 38 respectivelyrepresent an inlet port of the charge pump, and an inlet portcommunicating with the inlet port 455 of the charge pump.

As described above, the charging passage 433 has the second endcommunicating with the first pair of hydraulic passages 431 a and thesecond pair of hydraulic passages 431 b via the check valves 461 a, 461b, 461 c and 461 d so as to allow the pressurized hydraulic fluid to befed from the common charging passage 433 into a lower pressure line ofthe first pair of hydraulic lines 484 a and a lower pressure line of thesecond pairs of hydraulic lines 484 b, while preventing the pressurizedhydraulic fluid from flowing in the reverse direction.

Further, bypass lines 462 a and 462 b having throttle valves are formedbetween at least one of the first pair of hydraulic passages 431 a andthe charging passage 433, and between at least one of the second pair ofhydraulic passages 431 b and the charging passage 433 (see FIGS. 33 and39).

The bypass lines 462 a and 462 b are designed to assure theneutralization of the hydraulic pumps 410 a and 410 b. Specifically,even if the swash plates 414 a and 414 b of the hydraulic pumps 410 aand 410 b tilt from the neutral positions by a small angle, there occursthe pressure difference between the first pair of hydraulic lines 484 a,and/or between the second pair of hydraulic lines 484 b. This pressuredifference causes the rotation of the hydraulic motors 482 a and 482 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 414 a and 414 b due to assembling errors or the like causes anunintentional rotation of the hydraulic motors 482 a and 482 b. On thecontrary, the bypass lines 462 a and 462 b, as described above, allowthe pressurized hydraulic fluid to leak therethrough from the first pairof hydraulic lines 484 a and the second pair of hydraulic lines 484 b.Thus, the pressure difference between the pair of first hydraulic lines484 a and/or between the second pair of hydraulic lines 484 b caneffectively be limited, thereby effectively avoiding the unintentionalrotation of the hydraulic motors 482 a and 482 b, even for the swashplates 414 a and 414 b having the actual neutral position displaced fromthe design neutral position due to the assembling errors or the like.

In view of transmission efficiency between the hydraulic pumps 410 a,410 b and the hydraulic motors 482 a, 482 b, the leakage of thepressurized hydraulic fluid from the first and send pairs of hydrauliclines 484 a, 484 b through the bypass lines 462 a, 462 b is notpreferable. Therefore, the bypass lines 462 a, 462 b are preferablyprovided in portions from the charging passage 433 to one of the firstpair of hydraulic passages 431 a, and to one of the second pair ofhydraulic passages 431 b.

The check valves 461 a, 461 b, 461 c and 461 d are more preferablyprovided with release means 462 to forcibly bring the first pair ofhydraulic passages 431 a into communication with one another, and thesecond pair of hydraulic passages 431 b into communication with oneanother, if an emergency arises, as illustrated in FIG. 36. The releasemeans 462 are designed to easily move the vehicle, when the vehicle mustforcibly be moved or the vehicle wheels must forcibly be rotated by manpower or the like due to the disorder of the power source 480, thehydraulic pumps 410 a, 410 b or the like. Specifically, when the vehiclewheels connected to the hydraulic motors 482 a and 482 b are forciblyrotated with the first pair of hydraulic lines 484 a and/or the secondpair of hydraulic lines 484 b lying in the closing state, there occursthe pressure difference between the first pair of hydraulic lines 484 a,and between the second pair of hydraulic lines 484 b. As a result, thevehicle is hardly moved, or the vehicle wheels are hardly rotated. Onthe contrary, the release means can easily achieve the communicationsbetween the first pair of hydraulic passages 431 a, and between thesecond pair of hydraulic passages 431 b by mechanically releasing allthe check valves 461 a to 461 d. Whereby, the vehicle can easily bemoved by man power or the like.

As illustrated in FIG. 39, all the release means 463 are preferablydisposed in the same side of the center section 430, so that the linkmechanism linking these release means 463 for operation of the same canhave a simplified structure.

As described above, the pump unit 400 of this embodiment includes thefirst and second hydraulic pumps 410 a and 410 b, the center section 430and the housing 420, all of which are integrally connected together toconstitute a single unit 400 a. Accordingly, both first and secondhydraulic pumps 410 a and 410 b can be installed on the vehicle only bymounting the single unit 400 a on the vehicle, thereby achievingimproved efficiency in assembling the vehicle.

The pump unit 400 of this embodiment also includes a reservoir tank 485supportingly connected to the single unit 400 a, as illustrated in FIGS.34 to 36. The reservoir tank 485 is designed to reserve the hydraulicfluid to be replenished to the first pair of hydraulic passages 431 aand the second pair of hydraulic passages 431 b. In this embodiment, thereservoir tank 485 has right and left sides respectively formingmounting ribs 485 a, through which the reservoir tank 485 is connectedto the single unit 400 a.

The above arrangement where the reservoir tank 485 is supportinglyconnected to the single unit 400 a can omit external conduits forfeeding the hydraulic fluid from the reservoir tank 485 to the chargepump 450, and external conduits for returning the hydraulic fluid fromthe single unit 400 a to the reservoir tank 485, thereby achieving easeof assembly, lower manufacturing cost, improved efficiency inreplenishing the hydraulic fluid through the decrease of the resistanceforce between the hydraulic fluid and the conduit wall, and producingother desirable effects.

The reservoir tank 485 preferably communicates with the housing 420 viaa hydraulic fluid communication passage 487 such as a pipe, asillustrated in FIGS. 34 and 35. This hydraulic communication allows bothreservoir tank 485 and housing 420 to be used as a hydraulic fluid tank,and hence the reservoir tank itself to have a reduced size. In thisarrangement, the reservoir tank is preferably located so that the upperlevel of the hydraulic fluid within the reservoir tank can be higherthan the upper end of the housing. This arrangement produces anadditional desirable effect, and more specifically allows the completefilling of the hydraulic fluid within the housing 420, therebyeffectively avoiding the air entrained in the hydraulic fluid. Inaddition, variation in volume of the hydraulic fluid within the housing420 due to variation in temperature of this hydraulic fluid can beproperly compensated by the reservoir tank 485 communicating with thehousing.

The reservoir tank 485 can be connected to the inlet port for charging434 via a hydraulic fluid replenishing passage 488 such as a pipe.According to the pump unit of this embodiment, which includes the chargepump 450 serving as the hydraulic fluid feeding means, as describedabove, the reservoir tank 485 communicates, via the hydraulic fluidreplenishing passage 488, with the inlet port 456 communicating with theinlet port 455 of the charge pump 450, and the outlet port 451 of thecharge pump 450 communicates with the inlet port for charging 434 viathe cartridge filter 486 (see FIGS. 34 and 37).

The communications between the reservoir tank 485 and the inlet port 434via the hydraulic fluid replenishing passage 488, and between thereservoir tank 485 and the housing 420 via the hydraulic fluidcommunication passage 487 can reduce the number of the conduits requiredrespectively between the first and second hydraulic pumps, and the firstand second hydraulic motors to substantially four conduits only,specifically the first pair of hydraulic lines 484 a and the second pairof hydraulic line s 484 b. Thus, as compared with the conventionalarrangements as disclosed in the above cited U.S. Pat. No. 4,920,733,the pump unit of this embodiment can achieve a lower manufacturing cost,an improved assembling efficiency and an excellent workability inmaintenance.

The pump unit 400 more preferably includes a cooling fan 481 disposednear the single unit 400 a and the reservoir tank 485 and operativelydriven by the power source 480. According to this arrangement with thecooling fan 481, the reservoir tank 485 is connected to the single unit400 a in such a manner as to form between the reservoir tank 485 and thesingle unit 400 a a clearance 489 into which a cooling air stream sdrawn from the cooling fan 481. The hydraulic fluid replenishing passage488 and/or the hydraulic fluid communication passage 487 traverses theclearance 489. In this arrangement, the hydraulic fluid replenishingpassage 488 and the hydraulic fluid communication passage 487 each mayhave the right and left sides surrounded by a cooling air duct or shroudto effectively guide the cooling air stream from the cooling fan to theclearance 489.

The thus arranged pump unit 400 can limit the increase in temperature ofthe hydraulic fluid stored in the reservoir tank 485 and the housing420, and also effectively limit the increase in temperature of thehydraulic fluid flowing through the hydraulic fluid replenishing passage488 and the hydraulic fluid communication passage 487. Thus, thetransmission efficiency between the hydraulic motors can be improved.

The hydraulic fluid replenishing passage and the hydraulic fluidcommunication passage each more preferably has an outer circumferenceprovided with fins (not shown) to obtain an enlarged heat radiatingarea, and hence an improved cooling efficiency. The fins can also beprovided on the reservoir tank 485 itself.

Preferably, the reservoir tank 485 is made of a semitransparent resinmaterial to afford a visual observation of the level of the hydraulicfluid from the outside of the tank. The reservoir tank 485 can alsoinclude a tank cap 485 b with an air release mechanism on the top of thetank.

In this embodiment, the charge pump 450 is provided to forcibly feed thepressurized hydraulic fluid to the inlet port for charging 434.Alternative to this arrangement with the charge pump 450, the inlet portfor charging 434 may be directly connected to the reservoir tank 485 viathe hydraulic fluid replenishing passage 488, thereby allowing thehydraulic fluid to spontaneously flow into the inlet port 434 when thepressure in a lower pressure line of the first pair of hydraulic lines484 a and/or the pressure in a lower pressure line of the second pair ofhydraulic lines 484 b drops from a predetermined value.

Eighth Embodiment

Another embodiment of the pump unit according to the fourth aspect ofthe present invention will be hereinafter described with reference toFIGS. 41 and 42. FIG. 41 is a longitudinal cross-sectional side view ofthe pump unit 400′, and FIG. 42 is a cross section taken along linesXXXXII—XXXXII in FIG. 41.

As illustrated in FIG. 41, the pump unit 400′ of this embodiment is atandem pump unit with the first hydraulic pump 410 a connected in serieswith the second hydraulic pump 410 b. In the following description,corresponding or identical parts to these of the seventh embodiment havebeen given the same reference characters or those with primes (′) toomit a detailed description thereof.

As illustrated in FIG. 41, the pump unit 400′ includes the commonhousing 420′ for accommodating the first hydraulic pump 410 a and thesecond hydraulic pump 410 b, and the first center section 430 a and thesecond center section 430 b respectively supporting the first hydraulicpump 410 a and the second hydraulic pump 410 b.

The common housing 420′ has the first end (the lower end in thisembodiment), and t e second end (the upper end in this embodiment) alongthe axial direction thereof respectively defining the first opening 420a′ for receiving the first hydraulic pump 410 a and the second opening420 b′ for receiving the second hydraulic pump 410 b.

The common housing 420′ also forms the partition wall 420 c′ atsubstantially the center in the direction of the pump shaft to dividethe common housing into the first pump accommodation chamber and thesecond pump accommodation chamber. The partition wall 420 c′ includes abearing portion for supporting the connection portion between the firstpump shaft 411 a and the second pump shaft 411 b. Specifically, thepartition wall 420 c′ includes a connection member 416 non-rotatablyfixed around the downstream end or the upper end of the first pump shaft411 a and the upstream end or the lower end of the second pump shaft 411b, and rotatably supported in the bearing hole 420 d ′formed in thepartition wall. The partition wall 420 c′ may form a plurality ofhydraulic fluid communication passages 420 e′ for communication betweenthe first pump accommodation chamber and the second pump accommodationchamber. These communication passages enable the entire housing to beused as the hydraulic fluid tank.

The first center section 430 a supports on the upper surface thereof thefirst hydraulic pump 410 a, and is connected to the housing 420′ in sucha manner as to seal the first opening 420 a′ of the housing. The firstpump shaft 411 a of the first hydraulic pump 410 a has the upstream endor the lower end extending downwardly through the first center section430 a to form a lower extension through which the power is inputted todrive the hydraulic pump units and the cooling fan 481.

On the other hand, the second center section 430 b supports on the lowersurface thereof the second hydraulic pump 410 b, and is connected to thehousing 420′ in such a manner as to seal the second opening 420 b′ ofthe housing 420′. The second pump shaft 411 b of the second hydraulicpump 410 b has the downstream end or the upper end extending upwardlythrough the second center section 430 b to form an upper extensionthrough which the charge pump 450 is driven.

The first center section 430 a, as illustrated in FIGS. 33 and 41, formsa first pair of hydraulic passages 431 a for the first hydraulic pump,respectively having first ends opening to the outside of the firstcenter section through the surface facing the first piston unit 412 a(the upper surface) to respectively communicate with the inlet/outletports of the first piston unit, and second ends opening to the outsideof the first center section. The second ends of the first pair ofhydraulic passages 431 a opening to the outside forms a first pair ofinlet/outlet ports 432 a respectively serving as connection ports forconnection with the first pair of hydraulic line s 484 a extending tothe first hydraulic motor 482 a.

Similarly, the second center section 430 b, as illustrated in FIGS. 33,41 and 41, forms a second pair of hydraulic passages 431 b for thesecond hydraulic pump, respectively having first ends opening to theoutside of the second center section through the surface facing thesecond piston unit 412 b to respectively communicate with theinlet/outlet ports of the second piston unit, and second ends opening tothe outside of the second center section. The second ends of the secondpair of hydraulic passages 431 b opening to the outside forms a secondpair of inlet/outlet ports 432 b respectively serving as connectionports for connection with the second pair of hydraulic lines 484 bextending to the second hydraulic motor 482 b.

Similarly to the seventh embodiment, the pump unit 400′ of thisembodiment includes the common charging passage 433 disposed therein,having a first end opening to the outside of the pump unit to form theinlet port for charging 434, and the second end communicating with thefirst and second pairs of hydraulic passages.

The common charging passage 433, as illustrated in FIGS. 41 and 42,includes a first bore portion 433 a, a conduit portion 433 b and asecond bore portion 433 c. The first bore portion 433 a is formed in thesecond center section 430 b to have a first end opening to the outsideof the second center section through the upper surface thereof to formthe inlet port for charging 434 and a second end communicating with thesecond pair of hydraulic passages 431 b via the check valves 461 c and461 d and opening to the second pump accommodation chamber. The conduitportion 433 b is disposed to have a first end connected to the secondend of the first bore portion 433 a and a second end extending throughthe second pump accommodation chamber, the partition wall 420 c and thefirst pump accommodation chamber to the first center section 430 a. Thesecond bore portion 433 c is formed in the first center section 430 a tohave a first end connected to the second end of the conduit portion 433b and a second end communicating with the first pair of hydraulicpassages 431 a via the check valves 461 a and 461 b. The conduit portion433 b can be extended through the partition wall 420 c′ by disposing theconduit portion 433 b within one of the plurality of hydraulic fluidcommunication passages 420 e′.

Connected to the common charging passage 433 is a pressure relief line453 communicating with the housing via a relief valve 452. The pressurerelief line 453, similarly to the seventh embodiment, is formed in thecharge pump casing 459 to communicate with the housing 420′ via thedrain port 435 formed in the second center section 430 b.

The thus arranged pump unit 400′ of this embodiment also produces thesame effects as those of the seventh embodiment.

Alternative to the conduit portion 433 b′, it is possible to form in theperipheral wall of the common housing 420 a communication hole having afirst end connected to the second end of the first bore portion 433 a′and a second end connected to the first end of the second bore portion433 c′.

Ninth Embodiment

One embodiment of the pump unit according to the fifth aspect of thepresent invention will be hereinafter described with reference to theaccompanying drawings. FIG. 43 is a hydraulic circuit diagram of thevehicle to which a pump unit 500 of this embodiment is applied. FIG. 44is a longitudinal cross-sectional side view of the pump unit and itsperiphery. FIGS. 45 to 48 are respectively cross sections taken alonglines XXXXV—XXXXV, XXXXVI—XXXXVI, XXXXVII—XXXXVII, andXXXXVIII—XXXXVIII.

As illustrated in FIGS. 43 to 45, the pump unit 500 is adapted to beused in a vehicle having right and left drive wheels 583 a and 583 b towhich first and second hydraulic motors 582 a and 582 b are respectivelyconnected, and includes a first hydraulic pump 510 a and a secondhydraulic pump 510 b respectively connected to the first and secondhydraulic motors 582 a and 582 b via a first pair of hydraulic lines 584a and a second pair of hydraulic lines 584 b, and a common housing 520for accommodating these hydraulic pumps 510 a and 510 b.

The connection form between the right and left drive wheels 583 a and583 b, and the first and second hydraulic motors 582 a and 582 b meantin this embodiment includes the direct connection of the drive wheelsrespectively to those hydraulic motors, and also an operative connectionof the drive wheels respectively to the hydraulic motors via a suitablepower transmission mechanism. In FIG. 43, the reference codes 580, 581and 586 respectively represent a power source, a cooling fan and afilter.

As illustrated in FIGS. 44 to 45, the first hydraulic pump 510 a and thesecond hydraulic pump 510 b are axial piston pumps of a variabledisplacement type, and respectively include a first pump shaft 511 a anda second pump shaft 511 b that have vertical axes and are disposedparallel to one another in the vehicle width direction within thehousing 520, a first piston unit 512 a and a second piston unit 512 bthat are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 513 a and a second cylinder block 513 bthat reciprocably support the piston units, a first angularly adjustableswash plate 514 a and a second angularly adjustable swash plate 514 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 515 a and a second control shaft 515 b thatcontrol the tilting angles of these swash plates.

The pump unit of this embodiment is of a vertical type with the firstand second pump shafts 511 a and 511 b having the vertical axes.However, the second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit500 can be of a horizontal type with the first and second pump shafts511 a and 511 b having the horizontal axes.

As best illustrated in FIG. 45, the first and second angularlyadjustable swash plates 514 a and 514 b of this embodiment are of acradle type.

As illustrated in FIGS. 44 and 48, the first control shaft 515 a and thesecond control shaft 515 b respectively have inner ends extending intothe housing 520 to be respectively connected to arms 516 a and 516 b andhence the first and second swash plates 514 a and 514 b, and outer endsextending rearwards in the vehicle longitudinal direction.

Alternative to the above arrangement, the first and second controlshafts 515 a and 515 b may extend away from one another in the vehiclewidth direction to respectively have oppositely positioned outer ends.This arrangement is advantageous when installed on the vehicle havingpush-pull control levers 198 a and 198 b as illustrated in FIG. 1, sincethe first and second control shafts 515 a and 515 b can have therotating shaft centers disposed parallel to the longitudinal axis of thecontrol levers, thereby achieving the simplification of a link mechanismbetween these control shafts and the control levers.

In the above arrangement, the first control shaft 515 a and the secondcontrol shaft 515 b are more preferably located at substantially thesame position with respect to the vehicle longitudinal direction. Thethus arranged first and second control shafts 515 a and 515 b can bealigned with the control levers in the vehicle width direction, therebyachieving a more simplified structure of the link mechanism.

The pump unit 500 further includes a common center section 530 thatsupports the first and second hydraulic pumps 510 a and 510 b, and apower transmission mechanism 540 that is accommodated within the housing520 to operatively connect the first and second hydraulic pump shafts511 a and 511 b together.

The pump unit 500 with the power transmission mechanism 540 permits thesimultaneous rotation of both pump shafts 511 a and 511 b only byconnecting the power source to either one of the first and second pumpshafts 511 a and 511 b, or to the first pump shaft 511 a in thisembodiment, thereby achieving the simplified structure for the powertransmission from the power source to the pump unit 500. In thisembodiment, the power transmission mechanism 540 is in the form of agear transmission device that includes a first gear 540 a non-rotatablysupported on the lower side of the first pump shaft 511 a, and a secondgear 540 b non-rotatably supported on the lower side of the second pumpshaft 511 b in meshed engagement with the first gear 540 a (see FIGS. 45and 46). Instead of the gear transmission device, any conventional powertransmission mechanisms such as chain and belt may be used.

The housing 520, as illustrated in FIGS. 44 and 45, includes a firsthousing 521 for accommodating the first and second hydraulic pumps 510 aand 510 b, and a second housing 522 for accommodating the powertransmission mechanism 540.

The first housing 521 has a box shape with a first sidewall 521 adisposed in the upper or lower side of the pump shafts 511 a and 511 balong the longitudinal direction thereof, or in this embodiment in thelower side of the pump shafts 511 a and 511 b, which will be hereinafterreferred to simply as the lower side, and a peripheral wall 521 bextending from a peripheral edge of the first sidewall 521 a to theopposite side of the pump shafts 511 a and 511 b along the longitudinaldirection thereof (i.e., the upper side of the pump shafts 511 a and 511b in this embodiment, which will be referred to simply as the upperside). The first sidewall 521 a forms bearing holes through which thefirst and second pump shafts 511 a and 511 b respectively extend. Theupper side of the first housing 521 has an end surface forming anopening through which the first and second hydraulic pumps 510 a and 510b can be placed into the first housing 521. The opening of the firsthousing 521 is sealed by the center section 530 in a liquid tightmanner. That is, the center section 530 of this embodiment constitutes apart of the first housing 521.

The second housing 522 is disposed in the lower side, and has a boxshape with a lower sidewall 522 a forming a bearing hole through whichthe lower end of the first pump shaft 511 a extends and a bearingportion for receiving the lower end of the second pump shaft 511 b, anda peripheral wall 522 b extending upwardly from a peripheral edge of thelower sidewall 522 a. The upper side of the second housing 522 forms anopening through which the power transmission mechanism 540 can be placedinto the second housing 522.

The second housing 522 is connected to the first housing 521 in such amanner as to have the opening sealed in a liquid tight manner by thefirst sidewall 521 a of the first housing 521, and form an accommodationspace of the power transmission mechanism 540 in cooperation with thefirst sidewall 521 a of the first housing 521.

In the thus arranged housing 520, the first sidewall 521 a of the firsthousing 521 serves as a partition wall dividing the accommodation spaceof t he housing into a hydraulic pump accommodation chamber and a powertransmission mechanism accommodation chamber. The partition wall thusdefining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 540 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 512 a, 512 b, cylinder blocks513 a, 513 b, and/or other parts. In addition to this foreign matterprevention measure, the first and second pump shafts 511 a and 511 b,which extend through the partition wall 521 a, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

Portions of the housing 520, through which the respective shafts 511 a,515 a and 515 b extend, are sealed by any suitable sealing means in aliquid tight manner, thereby allowing the housing 520 to serve as thehydraulic fluid tank.

The first sidewall 521 a serving as the partition wall preferably formsa hydraulic fluid communication hole (not shown) for communicationbetween the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber, with a filter provided inthe hole for preventing the intrusion of the foreign matters into thehydraulic pump accommodation chamber. The thus formed hydraulic fluidcommunication hole can omit the necessity of separately feeding thelubricant to the power transmission mechanism 540, with the result thatthe power transmission mechanism 540 can be lubricated with thehydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

In this embodiment, the first and second angularly adjustable swashplates 514 a and 514 b are of a cradle type, as illustrated in FIG. 45.Therefore, when the partition wall 521 a forms, on its side facing thehydraulic pumps 510 a, 510 b, spherical concave surfaces 524respectively adapted to spherical convex surfaces 519 formed in the rearsides of the swash plates 514 a and 514 b, which rear sides beingopposite to the surfaces facing the piston units 512 a and 512 b, thespherical concave surfaces 524 can slidingly guide the spherical convexsurfaces 519 of the swash plates 514 a and 514 b. The swash plates thuscan securely rest on the spherical concave surfaces 524.

In this embodiment, the first sidewall 521 a of the first housing 521serves as the partition wall. Alternatively, a partitioning means maytake various forms, as long as it can produce the same effect asdescribed above. For example, a separately prepared partition wall maybe mounted in a housing having a simple cylindrical box shape (see FIG.14).

Now, the description will be made for the center section 530. FIG. 49 isa cross section taken along lines XXXXIX—XXXXIX in FIG. 47. As bestillustrated in FIGS. 47 and 49, the center section 530 forms a firstpair of hydraulic passages 531 a respectively having first endscommunicating with the first piston unit and second ends opening to theoutside of the center section 530 to form a first pair of inlet/outletports 532 a serving as connection ports for connection with the firstpair of hydraulic lines 584 a (see FIG. 43).

The center section 530, as best illustrated in FIG. 47, also forms asecond pair of hydraulic passages 531 b having first ends communicatingwith the second piston unit and second ends opening to the outside ofthe center section 530 to form a second pair of inlet/outlet ports 532 bserving as connection ports for connection with the second pair ofhydraulic lines 584 b (see FIG. 43).

The common center section 530 thus forms all the first and second pairsof inlet/outlet ports 532 a and 532 b serving as the connection portsfor connection with the first and second pairs of hydraulic lines 584 aand 584 b. Whereby, the piping work between the hydraulic pumps 510 aand 510 b, and the hydraulic motors 582 a and 582 b can be facilitated.In this embodiment, the common center section 530 has side surfacesfacing one another that respectively form the first pair of inlet/outletports 532 a and the second pair of inlet/outlet ports 532 b.Alternatively, the first and second pairs of inlet/outlet ports 532 aand 532 b can be formed in the same side of the center section, therebyfurther facilitating the piping work.

The center section 530, as illustrated in FIGS. 44, 46 and 47, alsoforms a common charging passage 533 having a first end opening to theoutside of the center section 530 to form an inlet port for charging 534serving as an inlet for the hydraulic fluid to be replenished, and asecond end communicating with the first pair of hydraulic passages 531 aand the second pair of hydraulic passages 531 b via check valves 561 a,561 b, 561 c and 561 d.

In this embodiment, the first pump shaft 511 a, as best illustrated inFIGS. 44 and 45, has an extension extending upwards from the upper endthereof to be located above the center section 530, thereby supporting acharge pump 550 via the extension. The charge pump 550 has an uppersurface with a cartridge filter 586 detachably mounted thereto, throughwhich the hydraulic fluid is sucked into the inlet port 555 of thecharge pump 550. The cartridge filter 586 can be provided at thedischarge side of the charge pump 550.

The charging passage 533 is connected to a first end of a pressurerelief line 553 with a relief valve 552 therein. The relief valve 552regulates the hydraulic pressure of the charging passage 533 (see FIGS.43 and 44).

The pressure relief line 533 has a second end opening to the outside toform a drain port 554 through which the hydraulic fluid from the reliefvalve is drained.

In this embodiment, the pressure relief line 553 is formed in a chargepump casing 559 mounted on the upper surface of the center section 530.However, the present invention is not necessarily limited to thisarrangement. That is, the pressure relief line 553 can be formed in thecenter section 530.

Reference codes 551 and 556 in FIGS. 44 and 46 respectively represent anoutlet port of the charge pump, and an inlet port communicating with theinlet port 555 of the charge pump 550 via the filter 586.

As described above, the charging passage 533 has the second endcommunicating with the first pair of hydraulic passages 531 a and thesecond pair of hydraulic passages 531 b via the check valves 561 a, 561b, 561 c and 561 d so as to allow the pressurized hydraulic fluid to befed from the common charging passage 533 into a lower pressure line ofthe first pair of hydraulic lines 584 a and a lower pressure line of thesecond pairs of hydraulic lines 584 b, while preventing the pressurizedhydraulic fluid from flowing in the reverse direction.

Further, bypass lines 562 a and 562 b having throttle valves are formedbetween at least one of the first pair of hydraulic passages 531 a andthe charging passage 533, and between at least one of the second pair ofhydraulic passages 531 b and the charging passage 533 (see FIGS. 43 and47).

The bypass lines 562 a and 562 b are designed to assure theneutralization of the hydraulic pumps 510 a and 510 b. Specifically,even if the swash plates 514 a and/or 514 b of the hydraulic pumps 510 aand 510 b tilts from the neutral positions by a small angle, thereoccurs the pressure difference between the first pair of hydraulic lines484 a, and/or between the second pair of hydraulic lines 454 b. Thispressure difference causes the rotation of the hydraulic motors 582 aand 582 b. That is, even a slight amount of the displacement between theactual neutral positions and the predetermined design positions of theswash plates 514 a and 514 b due to assembling errors or the like causesan unintentional rotation of the hydraulic motors 582 a and 582 b. Onthe contrary, the bypass lines 562 a and 562 b, as described above,allow the pressurized hydraulic fluid to leak therethrough from thefirst pair of hydraulic lines 584 a and the second pair of hydrauliclines 584 b. Thus, the pressure difference between the pair of firsthydraulic lines 584 a and/or between the second pair of hydraulic lines584 b can effectively be limited, thereby effectively avoiding theunintentional rotation of the hydraulic motors 582 a and 582 b, even forthe swash plates 514 a and 514 b having the actual neutral positiondisplaced from the design neutral position due to the assembling errorsor the like.

In view of transmission efficiency between the hydraulic pumps 510 a,510 b and the hydraulic motors 582 a, 582 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 584 a, 584 b through the bypass lines 562 a, 562 b is notpreferable. Therefore, the bypass lines 562 a, 562 b are preferablyprovided in portions from the charging passage 533 to one of the firstpair of hydraulic passages 531 a, and to one of the second pair ofhydraulic passages 531 b.

The check valves 561 a, 561 b, 561 c and 561 d are more preferablyprovided with release means 562 to forcibly bring the first pair ofhydraulic passages 531 a into communication with one another, and thesecond pair of hydraulic passages 531 b into communication with oneanother, if an emergency arises, as illustrated in FIG. 47. The releasemeans 562 are designed to easily move the vehicle, when the vehicle mustforcibly be moved or the vehicle wheels must forcibly be rotated by manpower or the like due to the disorder of the power source 580, thehydraulic pumps 510 a, 510 b or the like. Specifically, when the vehiclewheels connected to the hydraulic motors 582 a and 582 b are forciblyrotated with the first pair of hydraulic lines 584 a and/or the secondpair of hydraulic lines 584 b lying in the closing state, there occursthe pressure difference between the first pair of hydraulic lines 584 a,and between the second pair of hydraulic lines 584 b. As a result, thevehicle is hardly moved, or the vehicle wheels are hardly rotated. Onthe contrary, the release means can easily achieve the communicationsbetween the first pair of hydraulic passages 531 a, and between thesecond pair of hydraulic passages 531 b by mechanically releasing allthe check valves 561 a to 561 d. Whereby, the vehicle can easily bemoved by man power or the like.

As illustrated in FIG. 47, all the release means 563 are preferablydisposed in the same side of the center section 530, so that the linkmechanism linking these release means 563 for operation of the same canhave a simplified structure.

As described above, the pump unit 500 of this embodiment includes thefirst and second hydraulic pumps 510 a and 510 b, the center section 530and the housing 520, all of which are integrally connected together toconstitute a single unit 500 a. Accordingly, both first and secondhydraulic pumps 510 a and 510 b can be installed on the vehicle only bymounting the single unit 500 a on the vehicle, thereby achieving animproved efficiency in assembling the vehicle.

The pump unit 500 of this embodiment also includes a reservoir tank 585supportingly connected to the single unit 500 a, as illustrated in FIGS.44 to 46. In this embodiment, the reservoir tank 585 has right and leftsides respectively forming mounting ribs 585 a, so that the reservoirtank 585 is supportingly connected to the single unit 500 a via mountingmembers 590 fastened to the mounting ribs 585 a.

The reservoir tank 585 communicates with the housing 520 for a freefluid communication therebetween via a hydraulic fluid replenishingpassage 587 or other suitable conduits means, as illustrated in FIGS. 44and 48. This hydraulic communication allows the reservoir tank 585 to beused as a hydraulic fluid tank together with the housing 520.

Preferably, the reservoir tank 585 has the upper side positioned higherthan the upper side of the housing 520, so that the housing can becompletely filled with the hydraulic fluid, thereby effectivelypreventing the air from being entrained in the hydraulic fluid storedwithin the housing 520. Variation in volume of the hydraulic fluidwithin the housing 520 due to variation in temperature of this hydraulicfluid can be properly compensated by the reservoir tank 585communicating with the housing.

The hydraulic fluid tank communicates with the inlet port for charging534. According to this embodiment, the hydraulic fluid, which has beensucked via the hydraulic fluid replenishing passage 588 from thereservoir tank 585 constituting a part of the hydraulic tank, is fedinto the inlet port for charging 534 via the pressure relief line 553(see FIGS. 43 and 44).

On the other hand, the drain port adapted to drain the hydraulic fluidfrom the relief valve 552 installed within the pressure relief line 553is connected to a cooling conduit 591 via a first end thereof. Thecooling conduit 591 has a second end communicating with the reservoirtank 585 constituting a part of the hydraulic tank. The cooling conduit591, as illustrated in FIG. 44, has at least a portion extending throughthe outside air with a spacing from the single unit 500 a and thereservoir tank 585 to air-cool the hydraulic fluid flowing through thecooling conduit 591. The cooling conduit 591 preferably has an outercircumference provided with cooling fins to obtain an enlarged heatradiation area, and hence improved cooling efficiency.

The cooling conduit 591 may be connected to the reservoir tank 585 orthe single unit 500 a by a suitable bridging means.

The pump unit is thus designed so that the hydraulic fluid, which hasbeen sucked into the inlet port 555 of the charge pump 550 via thehydraulic fluid replenishing passage 588, and discharged through theoutlet port 551, partly returns to the hydraulic fluid tank via thecooling conduit 591 extending through the outside air.

Specifically, the hydraulic fluid replenishing passage 588, a part ofthe pressure relief line 553 and the cooling conduit 591 togetherconstitutes a circulation line having a first end communicating with thehydraulic fluid tank and a second end again communicating with thehydraulic fluid tank. The charge pump 550 is also designed to allow thehydraulic fluid to be sucked through the first end of the circulationline and to be returned to the hydraulic fluid tank through the secondend of the circulation line. Whereby, the rise in temperature of thestored hydraulic fluid can effectively be prevented. As a result,deterioration in working efficiency of the hydraulic pumps and thehydraulic motors can effectively be prevented.

The pump unit of this embodiment has the arrangement to allow thehydraulic fluid drained from the relief valve 552 installed within thepressure relief line 553 to be returned to the hydraulic fluid tank viathe cooling conduit 591 in consideration of the cooling efficiency ofthe hydraulic fluid. That is, the hydraulic fluid discharged from thecharge pump 550 is highly pressurized, and therefore has a hightemperature due to the pressure energy of the discharged hydraulicfluid. Therefore, when the drained hydraulic fluid is directly returnedto the hydraulic fluid tank, the temperature of the hydraulic fluidstored in the hydraulic tank may increase. On the contrary, the pumpunit of this embodiment includes the cooling conduit 591 to return thehydraulic fluid of a high temperature drained from the relief valve 552to the hydraulic fluid tank to effectively limit the increase intemperature of the hydraulic fluid stored within the tank.

More preferably, the cooling fan 581 operatively driven by the powersource 580 is disposed near the single unit 500 a and the reservoir tank585, and the reservoir tank 585 is connected to the single unit 500 a insuch a manner as to form between the reservoir tank 585 and the singleunit 500 a a clearance 589 into which a cooling air stream is drawn fromthe cooling fan 581. The hydraulic fluid replenishing passage 588 and/orthe hydraulic fluid communication passage 587 traverses the clearance589.

In the above arrangement, the hydraulic fluid replenishing passage 588and the hydraulic fluid communication passage 587 each preferably havethe right and left sides surrounded by a cooling air duct or shroud toeffectively guide the cooling air stream from the cooling fan to theclearance 589.

In this embodiment, the mounting members 590 for mounting the reservoirtank 585 to the single unit 500 a is formed into a casing (see FIG. 50)with sidewalls, so that the cooling air stream from the cooling fan 581can efficiently drawn into the clearance along its sidewalls.

Such an additional fluid cooling arrangement can achieve cooling of thehydraulic fluid flowing through the hydraulic fluid replenishing passage588 and the hydraulic fluid communication passage 587 in addition to thecooling of the hydraulic fluid flowing through the cooling conduit 591,thereby more effectively limiting the increase in temperature of thehydraulic fluid within the hydraulic tank.

The hydraulic fluid replenishing passage and the hydraulic fluidcommunication passage each more preferably has an outer circumferenceprovided with cooling fins (not shown) to obtain an enlarged heatradiating area, and hence an improved cooling efficiency. The coolingfins can also be provided on the reservoir tank 585 itself.

Preferably, the reservoir tank 585 is made of a semitransparent resinmaterial to afford a visual observation of the level of the hydraulicfluid from the outside of the tank. The reservoir tank 585 can alsoinclude a tank cap 585 b with an air release mechanism on the top of thetank.

In this embodiment, the charge pump 550 is provided to forcibly feed thepressurized hydraulic fluid to the inlet port for charging 534, and tocirculate the hydraulic fluid through the circulation line. The presentinvention is not necessarily limited to this arrangement. Alternative tothis arrangement with the charge pump 550, the inlet port for charging534 may be directly connected to the reservoir tank 585 via thehydraulic fluid replenishing passage 588 by omitting the charge pump550. This allows the hydraulic fluid to spontaneously flow into theinlet port 534 when the pressure in a lower pressure line of the firstpair of hydraulic lines 584 a and/or the pressure in a lower pressureline of the second pair of hydraulic lines 584 b drops from apredetermined value. In addition, a pump may be separately provided tocirculate the hydraulic fluid through the circulation line.

The pump unit of this embodiment may employ the arrangement, whichallows a cooling air stream from a cooling fan (not shown) for the powersource or a radiator (not shown) to be applied on the cooling conduit591. This arrangement can more effectively cool the hydraulic fluidflowing through the cooling conduit 591.

Tenth Embodiment

Another embodiment of the pump unit according to the fifth aspect of thepresent invention will be hereinafter described with reference to FIG.51. FIG. 51 is a longitudinal cross-sectional side view of the pump unit500′.

As illustrated in FIG. 51, the pump unit 500′ of this embodiment is atandem pump unit with the first hydraulic pump 510 a connected in serieswith the second hydraulic pump 510 b. In the following description,corresponding or identical parts to those of the ninth embodiment havebeen given the same reference characters or those with primes (′) toomit a detailed description thereof.

As illustrated in FIG. 51, the pump unit 500′ includes the commonhousing 520′ for accommodating the first hydraulic pump 510 a and thesecond hydraulic pump 510 b, and the first center section 530 a and thesecond center section 530 b respectively supporting the first hydraulicpump 510 a and the second hydraulic pump 510 b.

The common housing 520′ has the first end (the lower end in thisembodiment), and the second end (the upper end in this embodiment) alongthe axial direction thereof respectively defining the first opening 520a′ for receiving the first hydraulic pump 510 a and the second opening520 b′ for receiving the second hydraulic pump 510 b.

The common housing 520′ also forms the partition wall 520 c′ atsubstantially the center in the direction of the pump shaft to dividethe common housing into the first pump accommodation chamber and thesecond pump accommodation chamber. The partition wall 520 c′ includes abearing portion for supporting the connection portion between the firstpump shaft 511 a and the second pump shaft 511 b. Specifically, thepartition wall 520 c′ includes a connection member 516 non-rotatablyfixed around the downstream end or the upper end of the first pump shaft511 a and the upstream end or the lower end of the second pump shaft 511b, and rotatably supported in the bearing hole 520 d′ formed in thepartition wall. The partition wall 520 c′ may form a plurality ofhydraulic fluid communication passages 520 e′ for communication betweenthe first pump accommodation chamber and the second pump accommodationchamber. These communication passages enable the entire housing to beused as the hydraulic fluid tank.

The first center section 530 a supports on the upper surface thereof thefirst hydraulic pump 510 a, and is connected to the housing 520′ in sucha manner as to seal the first opening 520 a′ of the housing. The firstpump shaft 511 a of the first hydraulic pump 510 a has the upstream endor the lower end extending downwardly through the first center section530 a to form a lower extension through which the power is inputted todrive the hydraulic pump units and the cooling fan 581.

On the other hand, the second center section 530 b supports on the lowersurface thereof the second hydraulic pump 510 b, and is connected to thehousing 520′ in such a manner as to seal the second opening 520 b′ ofthe housing 520′. The second pump shaft 511 b of the second hydraulicpump 510 b has the downstream end or the upper end extending upwardlythrough the second center section 530 b to form an upper extensionthrough which the charge pump 550 is driven.

The first center section 530 a, as illustrated in FIGS. 43 and 51, formsa first pair of hydraulic passages 531 a for the first hydraulic pump,respectively having first ends opening to the outside of the firstcenter section through the surface facing the first piston unit 512 a(the upper surface) to respectively communicate with the inlet/outletports of the first piston unit, and second ends opening to the outsideof the first center section. The second ends of the first pair ofhydraulic passages 531 a opening to the outside forms a first pair ofinlet/outlet ports 532 a respectively serving as connection ports forconnection with the first pair of hydraulic lines 584 a extending to thefirst hydraulic motor 582 a.

Similarly, the second center section 530 b, as illustrated in FIGS. 43and 51, forms a second pair of hydraulic passages 531 b for the secondhydraulic pump, respectively having first ends opening to the outside ofthe second center sect ion through the surface facing the second pistonunit 512 b to respectively communicate with the inlet/outlet ports ofthe second piston unit, and second ends opening to the outside of thesecond center section. The second ends of the second pair of hydraulicpassages 531 b opening to the outside forms a second pair ofinlet/outlet ports 532 b respectively serving as connection ports forconnection with the second pair of hydraulic lines 584 b extending tothe second hydraulic motor 582 b.

Similarly to the ninth embodiment, the pump unit 500′ of this embodimentincludes the common charging passage 533 disposed therein, having afirst end opening to the outside of the pump unit to form the inlet portfor charging 534, and the second end communicating with the first andsecond pairs of hydraulic passages.

The common charging passage 533, as illustrated in FIGS. 51, includes afirst bore portion 533 a, a conduit portion 533 b and a second boreportion 533 c. The first bore portion 533 a is formed in the secondcenter section 530 b to have a first end opening to the outside of thesecond center section through the upper surface thereof to form theinlet port for charging 534 and a second end communicating with thesecond pair of hydraulic passages 531 b via the check valves 561 c and561 d and opening to the second pump accommodation chamber. The conduitportion 533 b is disposed to have a first end connected to the secondend of the first bore portion 533 a and a second end extending throughthe second pump accommodation chamber, the partition wall 520 c and thefirst pump accommodation chamber to the first center section 530 a. Thesecond bore portion 533 c is formed in the first center section 530 a tohave a first end connected to the second end of the conduit portion 533b and a second end communicating with the first pair of hydraulicpassages 531 a via the check valves 561 a and 561 b. The conduit portion533 b can be extended through the partition wall 520 c′ by disposing theconduit portion 533 b within one of the plurality of hydraulic fluidcommunication passages 520 e′.

The charging passage 533 is also connected to the pressure relief line553 via the first end thereof with the relief valve 552 installedtherein in the same manner as the ninth embodiment. The relief valve 552regulates the hydraulic pressure of the charging passage 533 (see FIGS.43 and 44). The pressure relief line 533 has the second end opening tothe outside to form the drain port 554 through which the hydraulic fluidfrom the relief valve 552 is drained.

Connected to the drain port 554 is the cooling conduit 591, throughwhich the hydraulic fluid drained from the drain port 554 is returned tothe hydraulic tank, in the same manner as the ninth embodiment.

The thus arranged pump unit 500′ of this embodiment also produces thesame effects as those of the ninth embodiment.

Alternative to the conduit portion 533 b′, it is possible to form in theperipheral wall of the common housing 520 a communication hole having afirst end connected to the second end of the first bore portion 533 a′and a second end connected to the first end of the second bore portion533 c′.

This specification is by no means intended to restrict the presentinvention to the preferred embodiments set forth therein. Variousmodifications to the pump unit, as described herein, may be made bythose skilled in the art without departing from the spirit and scope ofthe present invention as defined in the appended claims.

What is claimed is:
 1. A pump unit comprising: at least one hydraulicpump with inlet and outlet ports formed therein; a pump case foraccommodating said at least one hydraulic pump, said pump case having anopening through which said at least one hydraulic pump is insertable; acenter section connected to said pump case in such a manner as to closesaid opening of the pump case; said center section forming a pair ofinlet/outlet passages having first ends respectively communicating withsaid inlet and outlet ports of said at least one hydraulic pump andsecond ends opening to the outside of the center section through a pumpcase abutting surface of the center section, and a first chargingpassage having a first end through which working hydraulic fluid is fedinto the center section and a second end opening to the outside of thecenter section through said pump case abutting surface of the centersection; at least one of said pump case and said center section forminga communication passage for communication between said second ends ofthe pair of inlet/outlet passages and said second end of the firstcharging passage; a first hydraulic fluid feeding valve for allowinghydraulic fluid to flow from said first charging passage to said pair ofinlet/outlet passages while preventing the reverse flow; and said firsthydraulic fluid feeding valve being installable through one of said pumpcase abutting surface of the center section and a center sectionabutting surface of the pump case.
 2. A pump unit according to claim 1,wherein: said pump case reserves the hydraulic fluid.
 3. A pump unitaccording to claim 1, wherein: said working hydraulic fluid fed intosaid first charging passage through the first end thereof is hydraulicfluid fed from a charge pump operatively connected to a driving shaftfor driving said at least one hydraulic pump; said center section formsa second charging passage for communication between the inside of saidpump case and said first charging passage; and said second chargingpassage prevents the flow of the hydraulic fluid from said firstcharging passage into said pump case, while allowing the hydraulic fluidreserved within said pump case to flow into said first charging passagewhen negative pressure is generated in at least one of said pair ofinlet/outlet passages.
 4. A pump unit according to claim 1, wherein:said communication passage has a groove shape and is formed in saidcenter section abutting surface of the pump case; and said pump case hassaid center section abutting surface forming an escape groovecommunicating with the inside of the pump case for the escape of thehydraulic fluid leaked.
 5. A pump unit according to claim 1, wherein:said center section forms a bypass passage for communication betweensaid pair of inlet/outlet passages; and said bypass passage is providedwith an open/close valve in such a manner as to be operable from theoutside for communication and cutoff of the hydraulic fluid between saidpair of inlet/outlet passages.
 6. A pump unit comprising: a firsthydraulic pump with inlet and outlet ports formed therein; a secondhydraulic pump with inlet and outlet ports formed therein disposedparallel to said first hydraulic pump; a pump case for accommodatingsaid first hydraulic pump and said second hydraulic pump, said pump casehaving an opening through which said first hydraulic pump and saidsecond hydraulic pump are insertable; a center section connected to saidpump case in such a manner as to close said opening of the pump case;said center section forming a first pair of inlet/outlet passages havingfirst ends respectively communicating with said inlet and outlet portsof said first hydraulic pump and second ends opening to the outside ofthe center section through a pump case abutting surface of the centersection, a second pair of inlet/outlet passages having first endsrespectively communicating with said inlet and outlet ports of saidsecond hydraulic pump and second ends opening to the outside of thecenter section through said pump case abutting surface of the centersection, and a first charging passage having a first end through whichworking hydraulic fluid is fed into the center section and a second endopening to the outside of the center section through said pump caseabutting surface of the center section; at least one of said pump caseand said center section forming a communication passage forcommunication said second ends of the pair of first inlet/outletpassages and the pair of second inlet/outlet passages to said second endof the first charging passage; a first hydraulic fluid feeding valve forallowing the flow of hydraulic fluid from said first charging passage tosaid pair of inlet/outlet passage and said second pair of inlet/outletpassages; and said first hydraulic fluid feeding valve being installablethrough one of said pump case abutting surface of the center section anda center section abutting surface of the pump case.
 7. A pump unitcomprising: a first hydraulic pump and a second hydraulic pumprespectively connected to first and second actuators via a first pair ofhydraulic lines and a second pair of hydraulic lines, said firsthydraulic pump and said second hydraulic pump accommodated within acommon housing, the former being disposed parallel to the latter; saidcommon housing having an end surface forming an opening through whichsaid first and second hydraulic pumps are insertable; said first andsecond hydraulic pumps supported on a common center section; said commoncenter section connected to said common housing in such a manner as toclose said opening while supporting said first and second hydraulicpumps; and said common center section forming a first pair ofinlet/outlet ports and a second pair of inlet/outlet ports open to theoutside of said common housing, said first pair of inlet/outlet portsrespectively serving as connection ports for connection with said firstpair of inlet/outlet hydraulic lines, and said second pair ofinlet/outlet ports respectively serving as connection ports forconnection with said second pair of inlet/outlet hydraulic lines.
 8. Apump unit according to claim 7, wherein: said common center sectionforms a common charging passage for feeding pressurized hydraulic fluidinto said first pair of hydraulic lines and said second pair ofhydraulic lines.
 9. A pump unit according to claim 7, wherein: saidfirst and second inlet/outlet ports are formed in the same side of saidcommon center section.
 10. A pump unit according to claim 7, wherein:said first hydraulic pump and said second hydraulic pump respectivelyhave pump shafts connected together by a power transmission mechanismprovided in said common housing; and said common housing includes apartition wall through which said pump shafts of said first hydraulicpump and said second hydraulic pump can pass, said partition walldividing said common housing into a pump accommodation chamber and apower transmission mechanism accommodation chamber.
 11. A pump unitaccording to claim 10, wherein: said first hydraulic pump and saidsecond hydraulic pump are of an axial piston type that include angularlyadjustable swash plates of a cradle type, said angularly adjustableswash plates respectively having rear sides forming spherical convexsurfaces, while said partition wall forming guiding surfacesrespectively sized and shaped to slidingly guide said spherical convexsurfaces of said angularly adjustable swash plates.